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Navigating the Twilight Zone: The Hidden Flaw in a Zoned Duct System

Hvac Zone Duct Dampers In SIP Structural Insulated Panel Cabin

Ah, so what exactly is this hidden flaw? “My HVAC guy put a zoned system in my house and told me it was the cat’s pajamas,” you may be thinking about now. Or maybe your HVAC installer described it as the bee’s knees, the eel’s ankle, or the elephant’s instep. Doesn’t matter. However they described it, there’s one piece they absolutely should NOT have installed.

First of all, let’s be precise in our language and clear up exactly what we’re talking about. The word zoning is used in more than one way in the context of heating and air conditioning systems in a house. First, larger houses are always zoned. That is, they have more than one thermostat so you can control the conditions separately in different parts of the house. If you have a two-story house, for example, you probably have at least two thermostats — one upstairs and one downstairs.

The other way that the term ‘zoning’ is used is to describe a single duct system attached to a single HVAC system that serves multiple zones. In most homes, each thermostat is connected to its own heating and cooling system. The home is zoned, but the HVAC system is not. In a ‘zoned system,’ a single heating and air conditioning system is controlled by multiple thermostats in multiple zones.

“Come on, man, just tell me what the flaw is!”

Hold on. Hold on. We’re getting there.

In the photo above, the three green lights are part of three zone dampers that control the flow of air to three separate zones. Depending on the needs of the house, any combination of 1, 2, or 3 zone dampers may be open and sending conditioned air to their respective zones.

If only 1 or 2 of hvac zoned system bypass duct air flowthe zones are calling for air, most air handlers will create extra static pressure because 1 or 2 of the pathways are closed off. Enter the bypass duct, shown at right. When the system is running but not all zone dampers are open, the bypass duct—in theory—is supposed to relieve the extra pressure and maintain good air flow throughout the duct system.

At the Affordable Comfort conference this year, I went to a talk on zoned duct systems where John Proctor and Rick Chitwood discussed the pros and cons of these systems. Proctor’s take is basically that zoned systems are horrible and shouldn’t be used. Chitwood likes them and says when done right, they provide exceptional performance.

On one point, though, they both agreed: Bypass ducts should never be used.

Here are three reasons why:

  • Throwing cold air directly into the return plenum reduces the temperature of the air coming in to be cooled. That makes the evaporator coil get colder, and the colder it gets, the less efficient it becomes.
  • The bypass duct steals air. Even with all three zone dampers open, the bypass duct has a big pressure difference across it, and air is lazy. It’ll cheat and take the path of least resistance whenever possible, in this case the bypass duct.
  • Not only is a colder evaporator coil less efficient, it’s also more likely to freeze up, as the condensation it collects eventually drops below the freezing point. (And if you think a bypass duct is bad for air flow, a frozen coil is way worse. It’s really hard to push air through a solid block of ice.)

The bottom line is that zoned duct systems are tricky. I do believe that Chitwood is right, but so is Proctor. I think Proctor’s main objection isn’t that zoned systems can’t work; it’s that they’re done wrong so often. In the end, if you do get one, make absolutely sure the installer doesn’t put in a bypass duct.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science.  He also writes the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


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This Post Has 122 Comments

  1. So how does a zoned system
    So how does a zoned system with no bypass deal with the excess static pressure?

  2. John N.:
    John N.: One way would be with multi-stage, variable speed equipment. Another would be spending lots of time testing and balancing the air flow in all modes. Yes, the extra air has to go somewhere, but you shouldn’t just dump it back into the return.

  3. The new Carrier Infinity with
    The new Carrier Infinity with Greenspeed Intelligence using a inverter compressor might be the ticket. Coupled with an Infinity zoning system, it has the potential to modulate capacity and match the load down to 40% of full capacity. So as long as the smallest load is 40% or more, it wont need a bypass. 
    A couple challenges need to be conquered before it becomes mainstream. 
    (1) Modulating airflow compromises the performance of the terminals.  
    (2) High Cost. 
    (3) No one at Carrier knows how to design a duct for it. (they do know the electronics) 
    (4) Bubba and Earl aint never gonna figger out how much freezone to put in this thing. 

  4. Danny G.:
    Danny G.: Yes, the Carrier Infinity system is pretty cool stuff. As you point out, though, it doesn’t come without some associated costs, and not just the monetary ones.

  5. Stick to the simple stuff.
    Stick to the simple stuff. All you have to do is set the dampers so that they do not close all the way 20-30% open and allow the excess air. It costs a couple of screws. Bypass the bypass, or never install one.  
    Unless you are going to reduce capacity to a “correctly done Man J plus 20% smaller plus or more,” and reduce the total CFM / sq ft of home, I dont see where a zone system is needed. 
    Here is why based on my experience: 
    I installed a zone system in a large condo once, and sized my AC unit 30% smaller than Man S, and designed the zones to follow the suns path, thinking it would be needed. It malfunctioned and when I arrived it was not working at all, as in all zones were calling. The house was at 65 degrees at 106 degrees outside. I realized then that if the system is balanced right, and sized right, that zones are irrelevant. I haven’t installed one since, and at the same time I have been pushing the sizing lower than the Man S calls for, just to see how far down the rabbit hole I can go, without zones. The lower I go the better the air equalizes temperature. Once I get to the bottom I may consider zones for special cases, like sun rooms, etc. 
    In fact I have been taking zones out of houses I redesign, and I have a nice collection of control boards and dampers piling up. LOL.

  6. Chris C.:
    Chris C.: So, you’re in the John Proctor camp, I see. Following the KISS principle is always a good way to go, and zoned HVAC systems aren’t so simple to do right.

  7. Like any technology, Air
    Like any technology, Air Zoning is often misunderstood and misapplied. The statement that all 3 dampers will be closed when the system is off is incorrect. The zone dampers default open when the HVAC system is off. It is true, unfortunately, that many bypass ducts are not properly designed nor installed correctly. When bypass sizing restrictions are followed and a means of setting differential pressure is installed into the bypass duct, the bypass flow will not adversely affect the HVAC system. Even the Carrier Infinity system will need a bypass, if too many small zones are created. In addition, zone systems should always have a Supply Air Sensor installed to protect against low Dx coil temperatures. Too often the contractor does not install it. There are many ways to manage airflow in a zoned system. A single method alone is insufficient. Those who rely upon the bypass alone are who get into trouble. There are many opinions out there. Just wanted to set the record straight.

  8. John B.:
    John B.: You are correct (at least partially): In the system shown above, the zone dampers did default to the open position. Not all do that, however. Some are normally closed when the system is off.  
    Regarding the use of bypass ducts, if John Proctor and Rick Chitwood say they’re a bad idea, that’s good enough for me. Those guys are two of the most knowledgeable people in the world when it comes to high performance HVAC.

  9. Mr. Proctor and Mr. Chitwood
    Mr. Proctor and Mr. Chitwood are trying to eliminate an effective airflow management tool. Why not just prohibit Flex-duct also. Very few contractors size and install flex-duct correctly, which is the primary root cause for poor airflow and efficiency losses in any HVAC system whether it is zoned or not. But Flex-duct will not be prohibited because it is a valuable tool used in many different applications. Contractors need Mfr’s support and guidance to properly design and install these systems. A properly sized and installed bypass duct is a valuable tool in airflow management.

  10. John B.:
    John B.: Flex duct can certainly create huge air flow problems, but in and of itself isn’t a problem. It can be installed in a way to get good static pressures and air flow, although, as you point out, it rarely is. I wrote an article on that topic a few months ago: Should Flex Duct Be Banned by Green Building Programs?. In fact, I’ve written many, many articles on ducts in this blog. 
    Now, back to bypass dampers: I agree that they can help with total external static pressure and maybe with air flow, but how do you suggest they be installed to eliminate the three problems I described in the article? I’m especially interested in hearing how you address the first one.

  11. Allison, here in lies the
    Allison, here in lies the problem with the internet and the “blogisphere”. Any homeowner can get on an uniformed rampage and spew information that they assume is accurate. But we all know what happens when we assume.  
    We should leave the HVAC stuff to the professionals. 
    Most information that is out on the internet is written with a “hidden agenda”. We should all check our information and the source. Just because “the most knowledgeable people in the world” say so on the internet, we should check with the professionals. 
    The uninformed are just as dangerous as the mis-informed.

  12. Larry: OK,
    Larry: OK, I’ll ignore the stuff about the homeowner on an uninformed rampage. You’re evidently reacting to something you don’t like without having done your homework. That’s fine. 
    And when you say, “We should leave the HVAC stuff to the professionals,” just who are the pros to whom you’re referring? The ones who did this? Or this? Or this
    If you have anything at all to do with residential HVAC, I advise you to find out who John Proctor and Rick Chitwood are.

  13. Allison I
    Allison I see you’ve come across one of my instructors who has taught several classes I’ve attended. Ask him about his house in Nevada County, CA!

  14. Yes, I’m referring to Rick
    Yes, I’m referring to Rick Chitwood. However, I too avoid zoned systems when I can.

  15. Hi, Allison, 
    Hi, Allison, 
    I AM an HVAC professional but do NOT play one on TV. Though I work almost exclusively in the commercial industry, I share your concerns with bypass ducts. We often find them completely out of commission soon after installation. In fact, I can’t say that I’ve ever found one to be operating completely as intended and have definitely never found a facility manager who likes their “poor man’s VAV” system.

  16. Allison, 

    Did you know, John Proctor supposedly holds a patent for a variable speed motor? By talking about eliminating By=Pass ducts he stands to gain something… 
    You mentioned I should do my homework…

  17. Dan K.:
    Dan K.: Ooh! What show are you on? I’ve gotta catch that one. :~) Regarding your comments on bypass ducts, I’m not surprised.

  18. Larry K.:
    Larry K.: If John didn’t already have an excellent reputation in the industry, and if the market for replacing bypass ducts with variable speed motors was anywhere near being almost, on the verge of, halfway significant, then that connection might raise some concern. My bigger concern at the moment, though, is to find out who the rampaging, uninformed homeowners are and if they’ve made an appearance here in the Energy Vanguard blog. ;~)

  19. Although I agree with JPB
    Although I agree with JPB that not all bypasses are created equal, I still think it’s a bad idea. Even the best implementation will have negative impact on system performance. There’s no getting around the fact that bypass air will move compressor away from its optimum operating point. A well-designed duct system and bypass will mitigate the effect, but there’s always an impact. 
    It’s not hard to accommodate single-stage zoning with without having to resort to bypass. My approach is a combination of upsized ducts and bleed-through (as Chris correctly advocated). Also, I make room assignments to zones according to similarity of load profiles. Once I have my airflow targets, I divide heating CFM by cooling CFM and look at ratios (ignoring rooms like halls and foyers where temperature swings are no issue). In my experience, this is not typically done and is a big reason many zoned systems perform poorly. 
    Equipment manufacturers support bypass because they don’t have control over duct design. Bypass has, for many years, been the status-quo for zoned system. Nevertheless, it’s nothing more than a band-aid. 
    @Chris, although I, too, prefer KISS, there are plenty of situations where a fixed system just can’t do the job. The obvious example is a two-story home with one HVAC system. North Carolina, and perhaps other states, now require zoning (or multiple systems) for multi-level homes. For small homes, separate systems is too costly and often adds too much capacity. And what about a large walk-out basement, where it’s not unusual for cooling load to be dominated by solar gain and heating load to be dominated by slab losses. In that case, without zoning, rooms with no above grade exposure will be over-cooled (or under-heated). 
    @Larry K, that sure sounds like an urban legend. In any case, Proctor is BY FAR not the only mechanical system expert who advocates against bypass.

  20. Allison, I already explained
    Allison, I already explained away your 3 points in my 1st response: 
    1. Monitoring the discharge temperature or Dx coil is mandatory. 
    2. Large bypass sizing is prohibited. Typically <25% of total Cfm.  
    3. A means of setting sufficient delta P, preventing the bypass from becoming the path of least resistance. 
    4. Last but not least. Contractor training and education is imperative, along with proper Mfr. guidance and support.

  21. David B.:
    David B.: Thanks for your excellent supplementary material to support what Proctor and Chitwood were saying.  
    John B.: No, you haven’t ‘explained away’ anything really. See David Butler’s comment right before yours. Don’t get me wrong. I’m not arguing against zoning because I think it can be useful. I just happen to agree with the sizable number of HVAC pros who say that bypass ducts shouldn’t be used.

  22. Just questions:  &lt
    Just questions: 
    1) Do all zoned systems either fully open or fully close their dampers? Or do some modulate the dampers? 
    2) Did someone hint at never fully closing any zone so that you never exceed a maximum pressure? (Essentially a bypass to all portions of the house rather than bypass back to the return side.) 
    3) Why not up-size the supply ducts so the pressure drops are low enough to eliminate the excess pressure problem?

  23. @Donald: 

    1) Some dampers can be modulated (requires a compatible controller), while others can be mechanically stopped at the desired minimum open position. 
    2) Yes, I referred to that as ‘bleed-through’ in my previous comment. 
    3) Also described in my above comment. 
    When designing a zoned duct system, I rely on a combination of (2) and (3) to ensure adequate airflow across the coil. If I have an especially small zone, I may gang it with a large zone, whereby a call from the small zone will open the large zone, but not vice versa. Plus I may overisze the ducts in small zones a bit more than the others. It’s always an exercise in give-and-take. Great fun. 
    @Allison, normally closed dampers would be a poor choice for residential zone control.

  24. David B.:
    David B.: I agree with you, but I’ve seen them in homes. It’s something you always have to pay attention to when you’re doing a duct leakage test.

  25. Disagree with proctor on
    Disagree with proctor on zoning being horrible. Example one: Homeowner decides to finish their basement on an existing home. 2 options are available: a) new system for basement or b) Zone from existing system conditioning the main floor. The new system option is expensive and generally way oversized as most basements have little to no load. The systems rarely runs and if it does the run time is not long enough to remove any humidity. Option 2 is to zone off main level. No by-pass is necessary as you can unload close to 100% of the cfm’s. Add (1) 8″ return and you get continuos air movement and de-humidification. There will be no negative pressure in the basement as you set the dampers to close 90% when no call from the basement zone, thus always supplying a small amount of air when the system is operating. 
    Example 2 has already been stated. Variable speed and properly sized ductwork to avoid the by-pass. I will try to find more info from proctor to see if i can learn anything new.

  26. Dewey N.:
    Dewey N.: I also think maybe Proctor’s throwing the baby out with the bath water on the issue of zoning. In some cases, as you showed, it just makes sense to zone a single system rather than throw another system in there. Thanks for your comment!

  27. It’s possible that Proctor is
    It’s possible that Proctor is simply recognizing the reality that most zoned systems are poorly designed. I always hesitate to specify zoning on a project unless I have control over who does the mechanical work.

  28. Allison, I am not sure I
    Allison, I am not sure I agree with your three reasons on why to never use a bypass damper. 
    1) With all zones open the bypass damper should be in the closed postion so there is no “stealing of air”. The bypass only opens as dampers close. 
    2)Coils should never freeze up when using bypass dampers becuase a discharge air sensor is used to prevent this from happening. 
    3)A colder coil temperature is not the only factor in the equation of energy usage. What about decreased equipment run time when using zoning with programmable thermostats? 

  29. “I realized then that if
    “I realized then that if the system is balanced right, and sized right, that zones are irrelevant. I haven’t installed one since, and at the same time I have been pushing the sizing lower than the Man S calls for, just to see how far down the rabbit hole I can go, without zones. The lower I go the better the air equalizes temperature” 
    Posted @ Friday, August 26, 2011 8:29 AM by Chris Cadwell 
    Chris, you and I see things from the same vantage point. Seems “undersized unit” is like a unicorn, I ain’t never seen one… 
    I do like and see advantage to communicating zoning, particularly in large homes.  
    When computers and printers became available typewriters became obsolete. Communicating zoning has done the same thing to the bypass damper. 
    Communicating zoning allows further “unicorn sizing” as it allows specific control of btu’s where they are wanted.  

  30. lots of posts so I hope not
    lots of posts so I hope not all of this is redundant. 
    Don’t let the homeowner talk you in to more than three zones 
    the zones should be as close as possible to equal cfm (don’t have one “small” zone. 
    leak 15-20% air to closed zones. tell the homeowner this is by design and not a problem 
    design the bypass properly, do not over-size it. you are not pushing air thru a duct, the bypass is being drawn into a negative pressure. 
    never route the bypass to the return plenum. always to a return air can or T-Y by the can. there should be 20 feet of duct between where the bypass air enters the return stream to mix before getting to the blower/fau. 
    if you are cycling on the DATS you’ve done something wrong and have trashed the system efficiency

  31. Many thanks to Allison for
    Many thanks to Allison for starting an interesting conversation.  
    To clarify a few items: 
    1) People confuse the situation by calling cooling capacity “excess air”. Single speed air conditioners draw almost the same watts whether you deliver all the capacity to the house or only some of it. With a bypass duct, you always hurt the efficiency of the unit. For example, a 25% bypass reduces the efficiency by 12%. Field studies and Carrier Corporation lab tests have shown the same thing. Larger bypasses are obviously worse.  
    2) When making a presentation at a conference, the title of the talk is intended to get people to come. The information at the talk is designed to help them make intelligent decisions. Ducted, dampered, zoned systems can work and can avoid their largest energy penalty by never using a bypass duct. Capacity and efficiency are too valuable to throw away.  
    3) We do not have a patent on a variable speed motor. We have worked with a US manufacturer to develop a BPM high efficiency motor that has built in algrythms to deal with dry, wet, and mixed climates. On a single speed (compressor) AC, we do not suggest substituting a variable speed motor for a bypass duct and lowering the speed of the motor to deal with a high static pressure when one or more zone dampers are closed. To do so would produce the same efficienty loss that occurs with the bypass duct. (It makes no difference if the air is resident in the coil longer than it should be because you pass it through multiple times or because you pass it through slowly). 
    4) Many or most houses do not need a zoned system for comfort. Those that do can be zoned to provide even comfort, even with ducted-dampered systems, without having a bypass duct. 
    5) There is no clear prohibition of bypass ducts that move more than 25% of the air.  

  32. I’m jumping in late because I
    I’m jumping in late because I’m researching ways to reduce capacity. I have a 500 sqft guest house in central Alabama with a 44K BTU 2-stage gas furnace & a 1.5 ton AC split system (too much heat, too much AC, & too much air). Is it absurd to consider using a bypass duct to improve dehumidification?

  33. I’m jumping in late because I
    I’m jumping in late because I’m researching ways to reduce capacity. I have a 500 sqft guest house in central Alabama with a 44K BTU 2-stage gas furnace & a 1.5 ton AC split system (too much heat, too much AC, & too much air). Is it absurd to consider using a bypass duct to improve dehumidification?

  34. The biggest problem with hvac
    The biggest problem with hvac zoning is design, design, design. 
    If you don’t design it right it won’t work. Controls are another issue. 
    If you are trying to solve a problem, the number one goal is to NOT introduce more 
    problems. (IE If you don’t know what you are doing you won’t pull zoning off.  
    I am all for zoning. I live in a 1700-1750 sq. ft. house in Katy, Texas area. My AC bill  
    typically runs around $30 a month for cooling areas I’m in running set points of 76 during  
    and 74 at night.  
    This is all documented on my website.  
    Before I zoned my house I was told how wrong I was… that it would never  
    work for such a small house.  
    The $30 a month a pay for ac is even with paying on average about 15 cents per KWH.  
    The ones who say it can’t be done will continue to say it because they 
    don’t understand it.  
    It’s always take the easy way out. 
    However, just because I can do it… doesn’t mean you will. 
    Design, design, design with controls being thrown in for good measure.  
    It does work. I am living proof. Look to the AC tips tab on my website… video proof is embedded. 
    I’ve been documenting my ac energy costs since 2010. The system was installed in 2007… yep there 
    is a learning curve, but I developed some proprietary skills in the process.

  35. Seems to me that the answer
    Seems to me that the answer to zoning is simple, and Carrier should have all the components to make it happen with there GreenSpeed product. 
    Variable speed compressor 
    Variable speed Fan 
    We need a Programmable controller which tells compressor and fan at which speed to run depending on what zones are active. Of course you would need to program in the CMF requirement for each zone and compressor should run at a speed that is appropriate for fan speed.  
    What Carrier needs to do now is to allow the compressor to power down lower than 40% ….. lets say 15% or 20% 
    So whats the problem? Getter Done!

  36. @Richard, the GreenSpeed (and
    @Richard, the GreenSpeed (and Bryant Evolution) only get to 40% on the 3 ton model. The 2 ton only has a 33% turn-down ratio. Caveat Emptor. Moreover, the system only comes in whole ton increments, and every BTU the system is oversized to the actual load eats into the dynamic operating range. So in practice, vs may not offer much of an advantage for zoning.

  37. David & Richard,&amp
    David & Richard, 
    Similar to a modulating furnace, the modulating compressor’s allowable control range is typically limited (perhaps 40% to 100%). Thus, if the Contractor creates 2-3 zones maximum, and the load for each zone falls within that (BTU capacity) control range, then the zone system could be quite effective at matching the system capacity to the momentary zone demand. The key is keeping the number of zones to a minimum and ensuring that all of the zones have similar load characteristics.

  38. Is it possible for a
    Is it possible for a Mitsubishi VFR system to be setup with a damper zone controlled system like the Carrier GreenSpeed zoning system? 
    I know the Mitsubishi VFR system, the compressors can power down to 15% of its capacity. If they can be setup like the Carrier zoning system you could better match the BTU requirements for each zone. I plan on having 3 zones maybe 4. 

  39. @Richard, are you referring
    @Richard, are you referring to City-Multi?

  40. From what i have been told,
    From what i have been told, Carrier is about to release a new HP/AC that works at 5 different speeds, with low speed being 20% of capacity. How the thermostat will control airflow will be the key to determining size of zones. 
    As for the Greenspeed, I have this system in my home with 3 zones. I have no idea exactly how they have the system programmed but have learned the following. It does not deliver capacity based on the load of each zone. Airflow limits are established by the ductwork capacity for each zone. Dampers have 15 positions and open and close based on how far each zone is out from thermostat set point. You can also alter airflow to a zone to some degree in the programming feature. Overall, it works well with zoning, just don’t expect it to deliver capacity based on the loads for a zone. Humididty, degrees out of set point, ductwork, and other factors determine capacity delivered. 
    Minimum airflow is as follows. This is how i would design ductwork for smallest zone. These numbers are based on heating airflow as it’s greater than cooling. 
    2-ton: 440 cfm 
    3-ton: 451 
    4-ton: 751 
    5-ton: 751 
    Basically, the smallest zone for a 2 or 3 ton system would be just over 12,000 btu’s or 1 ton of cooling. The 4 and 5 ton would be just shy of 24,000 btu’s. 
    If you install a duel fuel system, these minimum air flows do not work and typically will be much higher as you will need to design for the particular furnace installed. 
    Mitsubishi: Not 100% sure about this, but i don’t think you can use zone dampers. Zones are established by a variety of indoor units. Wall mounts, ceiling mounts, some small ducted zones, etc… You should be able to call them and they will help you with designing a system. Have Mitsubishi in my office, garage and room above garage in my home. Works great. Only problem is getting parts, expect delays.

  41. I am unfamiliar with that
    I am unfamiliar with that product but the Mitsubishi VFR system is probably operating on some hybrid/proprietary RS485 standard and would not be controllable via a 3rd party zone control product. Even if you could control it, doing so would most likely void the Mfr’s warranty.

  42. Mitsubishi does not make
    Mitsubishi does not make residential VRF equipment, nor does anyone else for that matter. VRF is not the same as a variable capacity compressor. With VRF, you can have heat in one zone and cooling in another. City Multi SEER is no better than low-end 2-stage or high end single stage (mid-teens).  
    @Dewey, your observation about zoning with mini-splits is correct. These systems are designed to support ‘equipment zoning’, not damper controlled zoning. Variable capacity mini-splits use proprietary controls to so you can’t overlay a 3rd party zone control system, at least not without disrupting the sophisticated compressor control technology. 
    @JPB, City Multi (Mitsubishi’s VRF line) uses R410a.

  43. Yep, that’s correct, these
    Yep, that’s correct, these mini-splits are classified as DX zoning not Air zoning. The term VRF stands for Variable Refrigerant Flow.

  44. Here is my recent experience
    Here is my recent experience with a zoned system, a prominent national builder and a client/buyer.  
    When a builder is putting in a dual zoned (single unit) AC system on a home, if the home is bigger than the HVAC system is designed to support on a single story home, is it possible that a smaller system that is designed to handle approximately 2000-2400 sf on one floor, could support a much larger home that is actually 2 stories? Let me say that a different way: Can one AC system support a 2 story home that is 2500 sf or bigger? 
    It is a somewhat complicated question (which the builders like, because they may try to lose you in unrelated facts about energy efficiency and over sizing of systems). Many factors go into the energy efficiency of a home, such as the insulation in the home, the R factor of the walls and ceiling, radiant barriers, doors and window use, number of people in the home, etc. That is all fine and good, but let’s talk about the real question: “Can a single system push enough cold air far enough, fast enough and hard enough to cool a larger home in a hot climate?” Without an answer to this question, the arguments of how well the home is insulated is of no consequence to the home owner. You can insulate a refrigerator to the highest degree possible, but if the coils and compressor in the refrigerator are not putting out adequate cooling, your food will still spoil. And we have hot summers in Texas–you don’t want your family to spoil in a hot home. 
    If you live in a state that depends on the use of your AC during the summer, this could be a big concern for you. I just recently went through an experience with a buyer and a new home builder (who builds hundreds or even thousands of new homes every year). Just before closing, we discovered that this home did not feel like it was cool properly, especially in the back of house–so, we started investigating the situation. The builder also started an investigation that consisted of two parts: (1) They bought out an independent inspector to look at air flow and leakage (sadly, they did not talk to the inspector to find out what the information meant and they did not use the data to determine if there was a flaw in the design of the AC system of the house. They did not use the report at all) (2) The asked the sub-contractor who installed the system and has possibly the most to lose if the system was not properly sized, to come and look at the system. The installed corrected the thermostats, that were monitoring the wrong floors (1st floor tied to 2nd, 2nd tied to 1st floor), but did not address the inadequate air flow that was discovered by the independent inspector. 
    We later ran our own set of tests. We set the AC to 77 degrees and left the house for a couple of days to be sure the air was really 77 degrees. Then we tuned the AC unit to 73 degrees on both floors and recorded the temperature every 15 minutes for 1 full hour. After approximately 30 minutes, both floors had dropped by 1 degree in temperature. After another 30 minutes only one floor had dropped an additional 1 degree and the other floor was still sitting at 76 degrees. Outside temperature was showing between 88 – 91 degrees. No a hot day by Texas standards. 
    I can tell you that if my AC would not cool my house by 2 degrees in 10 minutes on a real hot Texas day, my wife and kids would be telling me that I needed to get our licensed AC tech in to see what was wrong. And while we are on the subject of AC Technicians, I called my guys that I have been using personally for years and have been sending to clients for jobs from basis service to all new installations. For 20 minutes he railed on how these zoned systems are being sized as if the system is only cooling one floor at a time. So, if the house is 2800 sf, split between first and second floor, the system may be rated for just half of the size of the house. The assumptions here are (1) Only half of the house needs to be cooled at any one point in time (2) the duct work that goes to the second zone is short enough or designed so that air flow will carry at greater distances than normal to get to the secondary zone. Big assumptions. In our case, we took note of duct work in the same home floor plan and it appeared as though the builder’s AC company was putting in way too much and too large of duct runs for the air to get to the back of the house–which is where the greatest problem was easily noticed. 
    So, the next question I asked was to one of my trusted inspectors that I have worked with for years. He came from more than 20 years of construction experience before he started inspecting houses and he understands construction. I asked him what he thought of zoned AC systems and he said that he had no issue with them if they were zoned properly to the size of the home (not just sized to one floor). He said what my AC guy said and what seems to be a generally accepted engineering spec for sizing AC systems from any manufacturer or SEER rating, that 1 ton of AC per 500-600 sf of house is the rule of thumb that I have never seen disputed. So, if your house is 2500 sf, you would generally need about 5 tons of AC capacity to heat and cool the house. Literally, to blow enough air in the house. Of course, that has to be distributed correctly to the floors according to sf of the floor and size of the system. 
    Now, let’s talk about builder diversions to the engineering questions:  
    (1) “Over-sizing is a big problem!” Yes. Over-sizing of a system can be a problem. I have never seen a system that is over-sized, but I am told that it can create some of the same problems of humidity control and electrical use that an under-sized unit can create. So, again, we fall back on the questions of “What is the fundamental engineering spec for an AC system as expressed in AC size to SF of the house and does this house meet that basic specification?” and “Can your builder document and prove that the system is sized properly to the size of the house 
    (2) “Our insulation is better than most homes!” I have heard this from builders for the past 7 years from every builder salesperson in Austin. They all claim that their methods are unique and better, when in fact the details of what they are doing are almost all identical. There are some exceptions, but buyers need to keep in mind the example of the refrigerator. If you can not move enough air across the house to begin with because the system is not strong enough or the duct work is too long and poorly designed, you can have a 5 start green/no air leak/100% sealed home, and it still would not cool the home. Insulation is not enough to cool a house. (And the truth is, most builders are not doing anything different today with their windows, doors, radiant barrier and attic insulation than they were 3-4 years ago when they were not using zoned AC systems. Check it out. It is the same. 
    (3) The computer said it was okay. This one is the real leap of faith. This is probably the first thing that you will hear, too. “Oh we don’t use engineering spec’s anymore. We use a really smart computer program that tells us how to rate the system.” Well, if this is true, then the program should be smart enough to tell us how much air flow is needs in each room according to the cubic square feet of the room, how many windows are in the house, which way the sun is coming in from the windows, how much air flow is needed in each room and now long each run of duct can extend without degrading the air flow in home or individual rooms. And then the air flow measurements that are conducted on the house (by people, not a computer program) should verify that in fact that much air is going into and cooling the room. Secondly, the systems should heat and cool the room at a similar rate as it does in any typical home (not 1-2 degrees an hour). Lastly, the house should feel like the temperature that it is set to, or there should be a valid engineering reason why it is not performing as designed. 
    (4) “Don’t worry about it. If it does not work, it is covered by the warranty and we will make adjustments.” This is the most dangerous thing you can possibly do because once you sign off on the house, getting something changed after the fact is next to impossible. Yes, maybe they will send a AC tech out to your house from the sub-contractor who will tell you that it is operating fine and maybe you should keep your doors shut, shades pulled down and spend more time in the part of the house that is coolest until summer is over. “After all, this is Texas! What do you expect?!” I hope you were not expecting that the AC company was going to redesign or change your system out for a larger one or two systems–that is not going to happen. 
    The concern and potential for short term problems is that if your system is under-rated, it will not cool your home and it will run for long periods of time to support minimal or moderate cooling of your home. And when the real heat of July and August hits Texas–oh my! Expect much higher energy bills and turn on those fans. You will need them. 
    The long term risk should be an even bigger concern. If your system is running at 2-5 times its normal duty cycle, you should expect to have to replace (and upgrade) your system in a much shorter period of time. Motored are designed to last X number of years based on engineering specifications of duty cycles. If you are running your AC system at 2x, 3x or 5x the normal duty cycle, you should expect higher maintenance bills, higher electrical bills and early replacement of the issue. This is not hard to figure out. Example: If your normal $25 hair dryer is designed to be used for 10 minutes a day and you used commercially in a hair salon for 10 mins x 10-20 times a day — it is probably going to die an early death, as compared to the hair dryer that you use once a day. 
    So, if you are buying a big new home and when you go outside you only see one AC system outside (compressor unit), maybe you should start asking some questions. 
    Mr. Builder: 
    How was this system designed? What is the engineering spec on the system? 
    How many systems are installed on the house and how is the air-flow distributed? 
    If using a single zoned system, how has it been sized and designed? What is the engineering spec? 
    If you have a computer program telling you it is okay, show us the data that was entered into the program for this house, all the assumptions of data entered into the program, the engineering spec of the calculations used and the air flow data as compared to how it is performing in this specific house.  
    While every home may have a design spec, not every installation is the same, so you should learn everything that you can while your house is being built and don’t assume anything. Ask more questions.  
    So, what happened to my buyer and builder situation? We asked too many questions that the builder and HVAC sub-contractor could not or would not answer. Rather than spending the money to fix or change the system, they released my client from their contract. That’s the good news. The bad news is that this builder is still building homes all over with the misguided belief that if the AC is working and some technician says that “the computer program says it is okay”, that is “good enough” for them to keep selling homes as fast as they possibly can.

  45. @T.Thornton, I don’t think
    @T.Thornton, I don’t think anyone will actually read your ridiculously long diatribe, but in scanning through it, I noted two glaring misconceptions…  
    First, the amount of AC capacity required to cool a home depends on many factors, least of which is floor area. Number of windows exposed to direct sunlight, insulation, especially quality of installation, thermal bypass, solar properties of windows, how leaky the house is, ducts located in hot attic, etc, can make huge differences from one home to another, even if they’re the same size.  
    So that 500-600 rule of thumb you mentioned is the enemy of good design since it ignores these differences. 
    It’s not uncommon for homes built to the latest energy code (and actually verified as such) to only need half as much capacity as the 500-600 rule-of-thumb. I even have clients who can cool more than 4,000 ft with a 2 ton AC… when it’s 100F outside. So it all depends. The only way to determine the correct size is with careful computer modeling. Unfortunately, as you alluded to, few mech contractors bother to enter all the correct take-offs and specs when they use the load calc software. 
    A zoned system can indeed handle a larger load than a non-zoned system, but we’re only talking maybe 10% to 20%, depending on the diversity of the zones.  
    In other words, if both zones have lots of south and west facing glass, there’s probably not much diversity benefit. But if one zone has lots of south/west glass and the other zone has mostly east and north glass, then there would be a rather large diversity benefit. Both zones are unlikely to be at peak load at the same time.  
    I hope this helps.

  46. One more thing I can’t leave
    One more thing I can’t leave unchallenged… you say an AC should be able to cool down 2 degrees in 10 minutes on a ‘real hot Texas day’… 
    That’s simply wrong and it’s exactly that expectation that drives hvac contractors to continue to oversize equipment. 
    If a system can do that, then, by definition, it’s oversized. The definition of a correctly sized system is one that can *maintain* the space at the indoor design temp on “a real hot Texas day”… (technically ASHRAE & ACCA standards define the outdoor design temp as the first percentile temperature, or the temperature above which accounts for 1% of the hours in the year, based on 20 year climate data). 
    That doesn’t mean a correctly sized system can’t handle hotter temps, or recover from setback on less hot days. First, there’s probably 10% positive bias (‘slop’) built into the load calc procedure. And second, there’s a significant lag between the daily high and its impact on indoor temp, due to insulation and mass of home. So a properly sized system should be able handle the load well more than 99% of the hours. During those rare record-breaking heat waves, perhaps the house will be two or three degrees above design for a few hours, every few years. No biggie. 
    Sure, we can install larger systems that can make even your wife happy, and maybe that’s more important. BUT, she should also be aware of all the downsides that go along with that (see the ‘7 Reasons’ article on my website). 
    Builders and hvac contractors have a responsibility to educate clients /before/ the fact, when it doesn’t sound like they’re trying to dodge responsibility for poor design or installation quality, which is usually the underlying cause when a new system can’t maintain the design temp.

  47. @ T. Thornton: Dear T. There
    @ T. Thornton: Dear T. There are multiple items in your comment that bear some response. First is the zoning issue. I strongly suspect that the reason the house is not getting cooled per your criteria is that the AC is actually delivering way less than that to the conditioned space. I strongly suspect it has a bypass that is partially or totally open. Bypasses rob the AC of capacity. 
    Here is a report on zoning and bypasses that goes into detail on what happens Perhaps start on page 23. This is an engineering/scientific report. 
    One thing that seems to confuse people is the difference between rules of thumb and engineering. Rules of thumb are not engineering. Estimating the probably cooling load on a house based in window directions, solar het gain coefficient, wall insulation, etc. is engineering. 1 ton per 500 or 600 square feet is not an engineering specification, it is a rule of thumb that is obviously wrong since an uninsulated home with bad windows will have a lot more cooling load than the same size home with good windows and insulation.  
    Most air conditioners are oversized. This makes it difficult to design a good duct system that will move the right amount of air and BTUs of cooling to each room. And yes as you suggest when the system is installed it should be checked to see if the design flow is getting to each room.

  48. Mr. Thornton,   &lt
    Mr. Thornton,  
    As Mr. Butler and Mr. Proctor have pointed out, a proper load calculation must be performed on a home in order to determine the heat loss/gain of the home, and size the BTU capacity of the HVAC system. Rule of thumb engineering does not work. The load calculation (ACCA Manual J) is a software program. The ductwork must also be designed (ACCA Manual D) to accommodate the air volume of that HVAC system. If the two design programs are not performed correctly, the HVAC system may not perform as intended, whether it is zoned or not. 
    The theory/practice of under-sizing a HVAC system because it is zoned is completely wrong. On a hot Summer day (all zones may be open) your home is no longer zoned, thus the home would need the full cooling capacity that was specified in the load calculation, no more and no less. 
    As Mr. Proctor points out, it would be a good idea to check if your duct system has a bypass duct. If it does, make sure the regulating bypass damper is not stuck open (which would starve the home of airflow). In addition, make sure the bypass duct has been properly designed and balanced, to prevent the bypass duct from becoming the path of least resistance. This link will send you to a document on proper bypass design and field setup.;
    However, based on the description of your zoned HVAC problem, it sounds like the typical Equipment/Duct Sizing issue, the industry struggles with on a daily basis. 
    John Phillip Brown 
    Chief Engineer 
    EWC Controls, Inc. 

  49. Zoning is about design,
    Zoning is about design, design, design. Utilizing a bypass which exceeds 25% of the system capacity indicates a poor design. If a bypass is needed then the application should be reviewed for zoning with equipment: not dampers. 
    The XV 1500 mentioned earlier was the predecessor to the all the inverter and modulating systems coming out today. Lennox Harmony, Carrier Infinity, and Trane all have staged equipment that if applied correctly does not need bypass.  
    John Jones

  50. This thread is the only
    This thread is the only active one I can find. I hope someone can help. My 3 thermostat multi-zone system went to fan mode in all zones. Someone suggested a reset of the A/H breaker. That didn’t work. Turning off two stats allows one zone to cool at low airflow, but turn on another stat and that zone cools very slowly but the first zone shuts down. Turning on all three stats throws the system back into fan only. What should I do?

  51. Typically, it does not matter
    Typically, it does not matter that your thermostats are running the fan continuously. Cooling & Heating demands should trump Fan only calls.  
    You must determine what Brand of Zone system you have.  
    Contact a qualified Technician, who can troubleshoot the system for you. 

  52. Now this is a lot of
    Now this is a lot of information to absorb. Hopefully I did not miss a post regarding preexisting systems. Here is my question. 5 year old 2000 sq ft town home with the contractor grade goodman. 1stat down stairs with a manual damper for balancing system from upstairs to down. I am wanting to add a zone damper so I don’t have to set the stat down 5 degrees every evening just to be able to sleep upstairs. Above the garage is a troublesome room.  
    From what I’ve read I could possibly drop fan speed to low when only one zone is calling and them to high speed when both are calling. Any other thoughts as I will still have pretty high duct psi with out a bypass of some sort.  
    I also noticed someone mentioned increasing duct size.  
    I could just dump that extra psi into the garage although I’m sure it would sweat my garage door.  

  53. Once you have a completed
    Once you have a completed load calculation it will be possible to determine if the existing system is a good fit for a zoning system. Finished rooms above a garage (FROG) are problematic if they are not on an independent zone. Oversizing duct does not solve issues. Getting the proper amount of airflow to the correct room at the correct time is the key. Very few HVAC contractors apply zoning systems on a regular basis; therefore, finding a contractor with this skill set requires a little homework.

  54. 1) Get someone to determine
    1) Get someone to determine if the duct-work is sized for a zoning system. (guessing it’s not) 
    2)Room above garage will always be an issue even with zoning, unless you have a separate zone just for this area. 
    3)Can’t drop blower speed to low and then go to high when both zones are calling. 
    4)Most importantly: Can’t make garage a dump zone for air. House will go negative and you could have major issues. 
    Without looking at system it’s impossible to make suggestions for anyone. There may be some solutions using variable speed compressor or 5 speed compressor with existing duct-work. Expect to spend a large amount of money to do it correctly. Other alternative is to design with by-pass which will cause other issues. If you live in ATL contact me for free estimate. 
    Good luck.

  55. DO NOT dump conditioned air
    DO NOT dump conditioned air into a garage, and never, never draw air back from the garage. Danger of carbon monoxide or other fumes entering the home. 
    There are EWC zone systems that can limit capacity based on a single zone calling. High capacity will be allowed when two zones call. The room above the garage is a problem, because it’s located above the garage. Excessive heat loss/gain. You need a load calculation and house survey.  
    Sounds like Dewey can take care of you. 

  56. Thanks everyone for your
    Thanks everyone for your insight. Very helpful in keeping me from spending money and either not working or causing other issues. In the long run going with wiping the slate clean and starting over will keep the wife and two daughters comfortable. Should get decent energy savings over a 5 yo 10seer system.  
    So thanks again for OP and all comments. 
    While brainstorming a possible solution I thought of this but cost started getting to point where it made more sence to replace system. I’d like to share it and get your thoughts. 
    Install zone board and dampers. Set dampers to only close enough to direct more air upstairs or down. Maybe 40%. Use no bypass. Add a TXV and possibly a receiver. Last step would be to set fan for continuous run keeping upstairs and downstairs air mixing and keeping up and down closer in temp.  
    But I’d rather put that money toward new higher effeciencancy unit that is designed specifically for my town home. 
    Thanks again.

  57. @Jeff, as much as I enjoy
    @Jeff, as much as I enjoy helping folks understand how these things work in this forum, you need to consult with a qualified designer rather than trying to figure this out yourself. In all likelihood, your duct system isn’t suitable for zoning, but the first thing would be to determine the static pressure in its current configuration, otherwise your thoughts about modulating the u/s airflow is just speculation. Chances are much better than even that your duct system is restricted to begin with. 
    As Dewey said, you can’t just reduce the fan speed to manage the pressure. Proper static is an indicator that you’re getting good flow across the coil, not an end in of itself. When you damper a main trunk, you’re going to increase the static, which will slow down the blower, and make your system less efficient, reduce the output, and could freeze the coil. 
    Running the blower continuously in cooling mode can lead to RH problems and will eat your lunch if you don’t have a high efficiency air handler with ECM motor. BTW, I’m not sure how you could have a 10 SEER A/C that’s only 5 years old since 13 SEER has been the Federal minimum since 2006, but anything less than about 14 SEER will have an inefficient PSC blower motor. 
    Finally, I disagree slightly with what others have said regarding “bonus rooms” over garages. The reason these rooms nearly always have heating and cooling issues is because the envelope isn’t detailed properly. It certainly doesn’t have to be that way. Whether or not the defects can be remedied in an existing home depends on access behind the kneewalls.

  58. Please help. We bought our
    Please help. We bought our 20 year old 3400 sq. ft. house last year. We have 3 zones and 4 dampers. Two operate together for first floor and two each for second floor. One thermostat in master for bed and bath…smallest zone. One thermostat in hallway for three bedrooms and hall bath. I’m slowly becoming an expert in this field, out of necessity. I am a stay at home mom in my ‘spare time’. 🙂 Our air conditioner died and now we are revamping our entire system. I cannot decide what is best. I could use some help. We have a gas furnace and central air currently on an old zoning control system. We are considering the Carrier infinity 98 furnace 100k BTU and would be the ‘maiden voyage’ guinea pigs on the carrier 19vs modulating air conditioning system. My reason for considering these is the we have static pressure concerns due to small insulated ductwork…sized 14×8. If only I knew then what I know now… Anyway, I do not want a bypass damper. I can’t explain exactly why but from everything I’ve gathered in my research…there can be other ways to remedy the airflow problems. Am I foolish in thinking so? Can anyone shed some light on the subject and alternative choices to consider. I have full manual j calculations that I can supply via email If that would help. If we went off Carrier brand, we are considering Trane systems…variable heat and single or two stage air with Arzel zoning. Any thoughts to share on all of the above would be most appreciated as I’m losing sleep over this decision!!!!! Thank you!

  59. The hidden flaw in zone
    The hidden flaw in zone systems is finding a contractor that has experience in them. 
    The other flaw is that they are a labor drain… there are contractors that think they can do it 
    until they do a zone system and have nothing but problems. When they find they can’t make money at it  
    they stop offering it and bash the idea of zone systems because ‘they got better things to do’. 
    If a company is a true solution provider, they will offer you a solution and work thru any problems with  
    whatever it takes mentality. Equipment brand doesn’t matter all that much when considering zoning… some manufacturer’s  
    have been getting into offering zoning products, but many times these “name brand” approaches are more costly when the  
    system breaks down. 
    You won’t find any real answers here because they bash the idea of a zone system out of the gate. A properly working zoned system 
    comes down to proper design. The design comes from the contractor doing the job… If I had a customer telling me how to do my job because  
    they think they know more than me… I would pass on the job or quote it higher full well knowing that you as a customer could become a liability 
    because you wanted me to do it your way thinking you know more than I do.  
    While that may sound harsh… you hire a professional so you don’t have to do it. The homework  
    you should be doing is looking for a contractor well versed in zone system design and not looking to ‘do it yourself’.  
    There’s pros and cons to everything. I have a 4 zone system in my house of 1750 sq. ft. which is over kill for a  
    house that size… but I did it to prove a few points… 1. That it works. 2. To reduce my ac bill (usually less than $30 a month) 
    3. Better overall comfort. 
    It won’t work with out the proper design. It takes me at least 16 hours of work to come up with design. Then another 40-60 hours to install  
    and work out any issues. This is what I do for a living. 
    I won’t tell you how to do it. Other than to say “I CAN DO IT”. 
    Your mileage will vary and vary greatly because 99% of the time you are  
    dealing with technicians and installers who don’t have the experience to  
    pull of zoning. Refer to: “we got better things to do”– because it takes  
    too much time and the problems it creates are hard to solve. 
    In other words… they aren’t solution providers are they?

  60. Ray A.:
    Ray A.: Perhaps you should go back and read the article. I was not bashing zoned systems. I was bashing bypass ducts.

  61. I would suggest to have the
    I would suggest to have the duct-work analyzed prior to making a decision. Under-sized duct-work will affect whatever decision you make. 
    The Manual J confirms the BTU capacity the home requires. Manual D is needed to confirm the duct can handle the air volume at a nominal static pressure value. 
    Can’t go wrong with a high end system like Carrier. If you want to eliminate the bypass you must select multi-stage or better yet modulating equipment. 
    Good Luck 

  62. @Cdnimo, sounds like you’ve
    @Cdnimo, sounds like you’ve done your homework, but variable capacity equipment isn’t a panacea and it certainly doesn’t eliminate the need for smart design. The modulating range is typically no better than 60% (40 to 100), and it’s much less for some models. For example, one of the Carrier Greenspeed VS models only has a 33% turn-down ratio. The marketing literature claims 60% turn-down, but only one model has anywhere close to that much range. 
    More importantly, in practice, much or all of the available range may be wasted if the system is oversized. I’d be willing to wager that most variable capacity equipment, as installed, operates at minimum capacity 99%-100% of the time, essentially acting like a very expensive single stage system, providing no advantage for zoning applications. 
    In any case, bypass may be necessary with a duct system that wasn’t specifically designed to counter the need for bypass. It’s true that bypass will impose a performance penalty for the reasons Allison said, but if designed properly, it can work. Although bypass is grossly overused and often poorly implemented, it’s nonetheless a useful tool when other airflow management methods aren’t possible. Whether or not bypass is necessary in your case depends on your ducts. 
    As my friend John just advised, you need to have your duct system analyzed. I would add that it needs to be someone like Ray who really knows their stuff. The question is how does a homeowner know whether a contractor is truly competent or simply has the gift of gab? One option would be to hire a 3rd party designer who could assist with vetting prospective contractors.

  63. David thank you for the very
    David thank you for the very kind words. 
    Allison I admit I didn’t read the article mainly because the 
    title lead me to believe what it was about. I read the comments and so 
    that was more or less what I was replying to. 
    Allison while you are correct that bypass is filled with danger, it can be done right. 
    It’s the exact opposite in regards to efficiency. When bypass is done properly you will gain 
    incredible efficiency.  
    If you drop the load on the compressor what happens to amp draw?  
    amp draw is what you are paying for.  
    With that said… if you do it wrong you wind up damaging equipment, equipment nuisance lockouts, duck work 
    blown apart and the list goes on and on. 
    What is wrong way and right way?  
    Somewhere north of 70 hours potentially. A lot of that work you don’t see me do. 
    In my experience 99% of zone systems are designed poorly or just plain wrong and will 
    never operate properly unless you start over from scratch.  
    It is a lot of work but the difference is night and day. 

  64. Ray wrote: “When bypass
    Ray wrote: “When bypass is done properly you will gain incredible efficiency.” 
    While I agree with everything else you wrote, this idea that bypass improves efficiency is wrong. If you meant that zoning done-right-can reduce energy consumption, then I agree. But bypass does not improve system efficiency. A properly designed zoned system without bypass will be more efficient than one with bypass. 
    First, entering dry bulb temperature has no impact on compressor watts. However, the bypass air may have a lower wet bulb temperature than return air from the house. This may have a measurable, but small impact (reduction) on compressor watts (typically less than 1%). However, output will drop even more, so efficiency end up taking a significant hit, manifesting in longer run times.  
    Second, sensible efficiency takes an even bigger hit than total efficiency since a cooler mixture of return air will lower the coil temp and thus the sensible ratio.  
    These factors translate into higher energy bills.

  65. This is what makes zoning a
    This is what makes zoning a product of the ‘wild west’. 
    I have my experience and you have yours.  
    I have my methods of design and you have yours.  
    While I agree that a zone system without a bypass can work, in my  
    opinion it is not ideal. To have a bypass or not is a design decision…  
    the best I can say is: “it depends on a host of various factors” 
    Having a bypass does not significantly increase run time. 
    (This example below assumes the volumes of air that are mixing within the  
    return are equal to one another to keep things *simple* LOL In reality nothing  
    about this is simple. This also assumes you have a 20 degree drop across the coil)  
    If you have 80 degree air you are trying to cool. 
    Out the bypass comes 60 degree air to make another pass. 
    That 60 degree air mixes with 80 degree air. 
    To simplify example: 60+80= 140/2 = 70 degree air.  
    That 70 degree air after making another pass turns into 50 degree air. 
    that 50 degree air mixes with 80 degree air again. 
    50+80 = 130/2 = 65 degrees. 
    That 65 degree air makes another pass across coil 
    to and falls to 45 degree air.  
    80+45 = 125/2 62.5 
    The longer the unit runs the 80 degree return air temperature drops and  
    much, much faster than a normal air conditioning system.  
    This does not equate to longer run times. 
    A system without a bypass you can not make such comparisons.  

  66. Sorry but compressor amp draw
    Sorry but compressor amp draw does not drop as air is bypassed.Many believe this for some reason. EWC has tested this many times. It remains the same. Once again, bypassing works when the  
    bypass duct is balanced to limit the volume of air that can move thru the bypass. Otherwise, high volume bypass air will affect sensible efficiency. EWC has tested and confirmed that a low volume of bypass air can result in a minor improvement in latent cooling capacity, but excessive volumes of bypass air can adversely affect the sensible capacity. A procedure for balancing the bypass duct is available at

  67. @Ray, You’re analysis fails
    @Ray, You’re analysis fails to account for the loss of the bypass air from the supply air stream.  
    If a colder coil were more efficient, then manufacturers would design systems as such. You must realize that manufacturers have enormous regulatory and market pressure to improve system efficiency ratings.

  68. Wow — this will not go away.
    Wow — this will not go away. So I am making a demonstration video on bypasses. It should be up on our website at the end of Monday. I will keep you posted. 
    With respect to bypass air, any amount of bypass will lower sensible efficiency. If you want a unit to do more dehumidification, just slow down the airflow, you don’t need a bypass. 
    With respect to the cold coil question please consider the physics. (a graph of this will also go up late Monday): 
    The reverse Carnot cycle establishes a theoretical Coefficient of Performance (COPC) of a vapor compression air conditioner. The Carnot cycle is a clearly unattainable ideal, but it make two things perfectly clear: 
    1. Higher condenser temperatures reduce the efficiency of the air conditioner. 
    2. Lower evaporator temperatures reduce the efficiency of the air conditioner. 
    Thank you 

  69. “My analysis fails to
    “My analysis fails to account for the loss 
    of the bypass air from the air stream” 
    This just sounds like you are looking for a scab to pick at. 
    Bypass air is redirected back through the return… how you call this a loss is beyond me.  
    If a zone only needs 1 ton of air isn’t this what you should deliver to it? I never said anything about a cold coil… what I referred to was the faster drop of supply air temp. Cold coil and air temp is not the same thing… air temp is definitive cold coil is not. If the coil freezes it’s counter productive. If you shave run time you shave operation costs. Pros and cons to everything you do. No different if it’s me or you trying to do something. I face the same pros and cons. I don’t just sprinkle magic Austin dust on air conditioning equipment.  
    I have just this morning run some amp tests of my own system but I have yet to upload the pictures. What I saw from the tests (of my own system) is about 1 amp of savings toward the end of the call or cut out of the second stage compressor. Note: my situation is different than 99% of the units out there in that I have a 2 stage system. 
    That means 2 physical compressors. So the amp draws have a forced reduction due to  
    my system design. The amp draw from first to second stage is almost half. So if one zone is calling 2nd stage rarely if ever engages.  
    You can nit pick all you want… It costs me $30 a month to run my ac sometimes less. I have a neighbor who was paying 7 times this amount (just for ac). I have seen electric bills in comparison to the size of my house hitting as high as 10 times what I pay for air conditioning.  
    The bottom line is more comfort, less operational costs.  
    I have better things to do than argue over nothing. I will post the pictures of  
    my test here in a day or two to my website. The difference in what I say is it’s my design, my test… yet others want to point to a manufacturer built on selling what they make. They have a convoluted interest in what they say and do. This doesn’t mean what they offer won’t work or doesn’t work. I’m sure there’s more than one way to do it. Each with their own list of pros and cons. 
    I have proven my way works and works well…. after that what else is there? To argue about nothing? Sure.  
    BTW, my system turns 8 years old next year March.  

  70. Ok for those of you
    Ok for those of you interested in seeing this I have the pictures of the fluctuation in amp draws given various conditions of my AC zoned system.  
    In my opinion the savings in fluctuations will be minimal. Obviously the biggest gains in efficiency in this design is that the first stage is practically half of what 2 stage is. So I wouldn’t read too much into this, but this is more or less to prove the point that the amp draws will fluctuate much more than a regular air conditioning system that is not zoned. As demonstrated in the picture that shows all 4 zones running. When all 4 zones are running for this particular system it is running as a normal air conditioning system in that it is cooling the whole house rather than just a single or couple of different areas. 
    The biggest reduction in compressor amps drawn for this design is the forced reduction by using a full 2 stage air conditioning system. Due to this design operating costs are minimal in that if one zone is operated most of the time, the second stage will rarely if ever engage under that scenario. 
    Even still the bigger part of the gains in efficiency come from reducing waste and cooling only the areas of the home you are using.  
    If the link doesn’t show up above you can find the info by going to my website click on Air Conditioning Tips scroll down toward bottom of page and click on link titled “Do zone system ac compressor amp draw fluctuations save money?”  

  71. So, because the Bypass duct
    So, because the Bypass duct has been so over-used, misapplied and abused in the air zoning industry, the bypass has now become a practice that must be eliminated? Doing away with the Bypass completely is radical and extremist. Even California’s Title 24 allows approx. 12% bypass flow, so long as the Installer can prove 350cfm/ton return airflow in every zoned mode of operation.  
    Humidifier’s are frequently installed incorrectly and create the conditions for mold to occur ion the duct system. The fault lies with the Installer, not the humidifier. Why not eliminate humidifiers as well? Education, guidance and training on when and how to apply these devices is what the industry needs. 
    ACCA Manual Zr recommends a Bypass duct as one of several effective airflow management tools. Relying solely on the bypass is not advised, as has been the case in the past. The Zr manual (when properly applied) provides guidance for calculating and limiting the volume of bypass air to a value, that will not adversely affect efficiency ratios. 

  72. Ray, 

    Most certainly Amp draw goes down significantly in 1st stage cooling versus 2nd stage cooling. I am sure this is when you achieve the energy savings you mention. Your zone system may force the AC unit to remain in 1st stage, unless all zones are active.  
    But amp draw does not change significantly as “air is bypassed”. Your ammeter readings (2nd stage @ 4 zones and 2 zones) are close enough, to be considered within the tolerance range for any digital meter.  
    The difference is negligible and thus, difficult to use when calculating savings. Shorter run time and 1st stage cooling however, is where you are getting your incredible savings.  
    Zoning a 2 stage AC system is the big advantage you have over your neighbors. 

  73. @Ray, you seem to equate amp
    @Ray, you seem to equate amp draw with efficiency. Equipment efficiency = BTU’s produced divided by BTU’s (power) consumed. Similarly, overall system efficiency = BTU’s delivered divided by BTU’s consumed. 
    The omission of bypass air in your analysis is salient is because it represents a percentage of BTU’s produced that aren’t delivered to the space. 
    No one said your system isn’t efficient or economical, but you can’t credit bypass with any of that. Had you avoided bypass, your system would be more efficient. How much more depends largely on how much air you’re bypassing. 
    John Brown wrote: “So, because the Bypass duct has been so over-used, misapplied and abused in the air zoning industry, the bypass has now become a practice that must be eliminated?… Why not eliminate humidifiers as well?” 
    No, not eliminated. The cliche “the house is a system” is apropos here. People in the home performance industry often invoke the term without fully understanding all the implications or the opportunities it represents.  
    If there’s a need for mechanical dehumidification, it’s usually because something else in the system (e.g., ventilation, infiltration, bulk water management) is broken, or at least wasn’t specifically designed to control moisture at the source, which is always more efficient than removing it.  
    I see bypass in much the same way. It’s a tool that should only be used when options aren’t feasible or possible. 
    As for Title 24 and Manual Zr, I know you’re far too smart to actually believe that just because something is allowed in code proves best practice. It’s going to take a while to change the status quo.

  74. @David efficiency can be
    @David efficiency can be equated to multiple things… but if you have a compressor that is drawing 15 amps vs. 8 amps or 8 amps vs. 4 amps is easy to determine which of those draws is more efficient than the other provided that each is doing the same amount of work. If the difference considered is “only” amp draw doesn’t that relate to efficiency?  
    Efficiency can also be considered in cutting waste. If you have a structure in which you are only using part of, then there is no reason to cool the whole structure. Most of this started with commercial entities. They started figuring out due to threats of brown outs or the threat of random / rolling black outs if they didn’t reduce their drain on the grid.  
    Energy management systems were born and the controls were taken away from those on the manufacturing floor, office cube, empty conference room etc.  
    While a difference of 1 amp here or there won’t amount to much initially over time it does add up. 
    “I can’t credit bypass with any efficiency.” Well that is a loaded question for someone who has installed a zone system vs. someone who to my knowledge has not. If you have where are your records?  
    Does a bypass account for all the efficiency? Uh no. Like I’ve said countless times before the majority of the efficiency comes from cutting waste. The bypass is just a way to control air flow that is not needed— that’s all it’s ever been for IMHO… a control.  
    My air conditioner is a rated 18 SEER. (stamped on the unit when I bought it.) Before I started tweaking I could only achieve around 21 SEER this was 2010 time frame and documented on my website as well as additional years later in which I hit 54.5 SEER. Trade secrets were learned because of this event.  
    Now while this SEER rating is skewed because I figure it based on my operating costs it is merely a measurement. It is skewed only in the consideration that I rarely run all 4 zones. I would figure it no differently if I went to a customers home to determine the efficiency of their system if they asked me to do so. Most customers don’t have zone systems. 
    So to say that bypass accounts for all the efficiency no… does it help? I believe so.  
    To say that I could increase efficiency merely by doing away with the bypass?  
    What pray tell am I going to do with all that air?  
    Dump it into another zone that is not being used? Sure. when the purpose for putting in a zone system was so that I wouldn’t cool other parts of the house I am not using? What’s the purpose of putting in a zone system then? 
    David, I challenge you to put in a HVAC zoned system in your house… do it your way and we can compare notes. I have put my money where my mouth is… which is rare in this business from what I have seen thus far. 

  75. I do not advocate using a
    I do not advocate using a bypass purely for dehumidification nor as the sole solution to airflow management.  
    The affect bypass flow has on a DX cooling system is well known by some and not by others. I simply stated a fact that is known by testing and observation.  
    Similarly, the affect of too much bypass flow is known by some and not by others. 
    In my corner of the galaxy, EWC is constantly and continually providing best practice guidance to Contractors and End Users BEFORE a final decision is made. 
    As usual, my esteemed colleague Mr. Butler speaks wisely. 

  76. JPB, 

    If I ran my air conditioner as a normal one with  
    all zones open at 10.6 cents per KWH and keep temp  
    at 75 degrees through out the house it would cost me  
    around $67 a month given the rated efficiency of 18 SEER. 
    This is what my unit is rated for all things considered including  
    the 2 stage functionality. 
    Is this what I do? No. I zoned it. I run 1 zone all the time… and maybe 
    10% of the time I run an additional zone.  
    What I pay for air conditioning most months is less than $30 a month. 
    The efficiency comes from 2 places. 
    1. The rated efficiency of the unit. 
    2. The cutting of waste.  
    If I am saving $37 a month at current energy rate. 
    The zone system pays for itself. 
    The other is more comfort than you can imagine.  
    I would zone it the same way again even with all the 
    squabbling on this board. LOL. 
    Ok time to go buy a pet rock.

  77. David, 

    My personal opinion is that “one thing” whether it be amp draws, bypass, system design or whatever doesn’t necessarily decide for you how successful or unsuccessful you will be when it comes to zone systems. 
    Often times what I see is that people try to pin their problems on one aspect of design.  
    It’s the combination of the entire system in question that pulls it off… not just one aspect of it.  

  78. Ray, 

    I think all of us are simply trying to provide advice/best practice for others who are interested in zone systems or have had problems with a zone system.  
    Your passion and exuberance when discussing these systems is evident in your blogs.  
    ACCA’s Manual Zr is now available, which levels the playing field and provides superior zone system design guidance.  
    This guidance may be followed, in the absence of specific Mfr’s design guidance, which may not be clear and concise. 

  79. Dear Mr. Brown: Unfortunately
    Dear Mr. Brown: Unfortunately, I do not find Manual Zr clear and concise. It is a 274 page document that is difficult to follow. I know a lot of work was put into that document, but in the end it is confusing. To be clear neither Zr or Title 24 specify a maximum amount of bypass air flow. Zr does limit “dump zone” air to 15%. Manual Zr also says “limit values 
    can range from less than 10% to 90% of the blower Cfm”. 

  80. No Trailer Available.
    No Trailer Available. Unfortunately it takes hours to upload the demonstration video. It will be uploaded tomorrow morning. 
    Thank you, John

  81. JPB,  

    In my professional opinion advice on a forum board 
    doesn’t go far enough and it’s impossible to go far enough 
    with a subject like HVAC with zoning mixed in for “fun”.  
    Not only are there design challenges in zoning there 
    can many times be architectural challenges as well. 
    Home builders will slap up just about anything and 
    in many cases leave the HVAC contractor years later  
    with visions of taking someone out to the shed for a  
    little instruction “country style”. LOL. 
    Too many times blame is cast on 1 design flaw, and sure 
    in some cases that blame may be warranted, but using blanket 
    statements to cast blame on various aspects of a zoning system 
    is in my opinion not wise without looking at the entire 
    HVAC system.  
    Home owner education is a good thing, but the thinking that  
    one thing is “always” the problem is misguided at best no matter 
    how well intended the intent is or was.  

  82. Dear Mr. Proctor, 
    Dear Mr. Proctor, 
    I agree that Manual Zr is not light reading but neither is Manual D nor any ANSI standard that Contractors & Engineers should be familiar with.  
    The 10% – 90% bypass limit you mention in Zr (page 61), refers to a theoretical momentary bypass factor value, dependent upon momentary conditions. The attributes of the equipment (1 or 2 stage), ambient conditions, internal load, etc.  
    Zr does not advocate 90% bypass air. Zr simply provides BPF charts (page63) based on the OEM low limit dry-bulb leaving air temperature and outside air dry-bulb temperature conditions.  
    The more advanced settled air temperature equation (page 64) can be used to determine the acceptable bypass Cfm value for any set of operating conditions. 
    You are correct in that Title 24 does not specifically stipulate 12% maximum bypass volume, but it logically infers that value upon any single stage DX cooling system that is zoned with a bypass.  
    For example, Title 24 requires a field verified 350cfm/ton return airflow in every zonal mode of operation (in order to avoid an energy credit penalty).  
    Based on that requirement (on a typical 3 ton system @400cfm/ton), you would need to restrict/limit bypass flow to 150cfm maximum in order to comply. That equates to a BPF value of 12.5% (1200 x .125 = 150)(1200 – 150 = 1050/3 = 350). 
    Do you recall our visit to the test home in Rancho Cordova? 

  83. @John, thanks for putting
    @John, thanks for putting that video together. It will help enormously when I try to explain to contractors why my specifications prohibit bypass.

  84. Sorry John, you didn’t
    Sorry John, you didn’t convince me. 
    Nice idea as a test setup though. 😉 
    If I hadn’t gone out and installed my own nearly 8 years ago 
    you might had won me over at that time… especially the first 
    3 years of the system barely getting over the efficiency of the 
    rating of the ac equipment that I installed. That all changed  
    after 2011… I document all my energy bills on my website  
    since 2010. Even this year looks like another record breaker  
    so far. In part due to a better utility rate. Still it’s crazy good!  
    John, If anything you can hang your hat on: it’s that zone  
    systems are tough as well as a serious time and labor drain for  
    anyone who chooses to take a shot at one.  
    I’m sure this blog will live on in infamy because I get these so  
    called zoning gems that thought bypasses were dumb. There are 
    just as many problems with no bypass zone systems than there are  
    with zone systems that have a bypass. No one can do either of  
    them properly when faced with a serious heat wave that lasts 
    a few days, weeks or longer. I can get them to work mind you…  
    but only with varying degrees of trade offs or concessions.  
    Provided that the zone system is actually functioning.  
    Stop that earthling! He’s stolen the P356 Bypass Modulator! LOL.  
    Ok, I’m off to pet my rock. 😉 

  85. Well Ray, I guess that proves
    Well Ray, I guess that proves nothing will change your mind. On a slightly different note, rather than designing zoned systems where the ducts are too small to carry the capacity to the zone(s) that are calling, it would be better to design a distribution system that was able to carry the full capacity of the unit to the zones that were calling, and if necessary put the “excess capacity” into one or more of the other zones that will likely need that capacity.

  86. Mr. Proctor, 
    Mr. Proctor, 
    Nice video that reflects a lot of systems out there, which is why the bypass has such a bad reputation.  
    Of course, you know I’ve already seen this demonstration before. 
    The EER drops because the Delta T goes down due to excessive bypass flow (cold coil).  
    The bypass needs to be balanced along with a means of limiting the volume of bypass air. Just like water or electricity, the air chooses the path of least resistance. 
    Other airflow management tools are missing, such as leakage into the inactive zone, which would help to minimize bypass flow. 
    If it was a two stage system, the indoor blower would not be in high speed and a well designed bypass would not have opened at all. 
    It is unfortunate that revelations in bypass design and field setup have taken so long. A far as I am aware, EWC is the only Zone Mfr. trying to educate Contractors on this issue. 
    You observed the operation of a zone system with a well designed bypass in Rancho Cordova.  
    Sir, I know you believe that any amount of bypass is unacceptable. But a bypass that limits flow/volume to a specified value, is better than the bypass on most jobs you come across. 
    I think Ray needs help finding my P356 Explosive Bypass Modulator! 
    Best Regards, 

  87. Mr. Proctor,   &lt
    Mr. Proctor,  
    Nordyne makes a fully variable speed product thru inverter technology.  
    I took a course on it a couple of years ago. The unique thing they did  
    was that the system would “know” how many zones were calling 
    and know how much air flow to provide to each zone.  
    During set up you would enter in I believe a minimum and maximum 
    air flow setting and the blower would ramp up or down to hit whatever  
    setting was desired for each zone. So with this type of system you  
    would always hit desired air flow with no need to bypass anything.  
    The compressor is also fully variable as well and would ramp up or down  
    given the demand or load and an electronic controlled TXV of all things.  
    This type of system has a lot of wow factor, but the cost and complexity to 
    set it up is quite a hurdle. Not to mention that if the system breaks down  
    or parts fail you are tied to the manufacturer that made it… including any  
    thermostats or sensors installed. It’s a proprietary system.  
    “IF” I were to do a zone system without bypass this is how I would want  
    to do it. But because of the cost risk and being tied to one manufacturer  
    for everything, I have decided this technology still has too many risks  
    currently. (Nordyne requires you to take the coarse to even be able to offer 
    this kind of system, the distributor for Nordyne relocated outside my area 
    which gives you a clearer picture of the risk I talk about.)  
    Inverters are very fragile in that a good clean power source is recommended.  
    Where I live there are numerous occasions of brownouts and power surges. 
    So there are always pros and cons to consider to any thing you want to do.  
    Explosive Bypass Modulator? Oh yeah the EBM! How in the world could  
    I have forgotten about that? LOL.  
    Ok, my pet rock is getting hungry now…  
    Looks like we’re having ‘rocky road’. 

  88. Ray, 

    Inverter drives are coming along and promise to be a great way to manage capacity/airflow in a zoned system.  
    EWC has just completed testing the Daikin Inverter AC, along with our UT3000 communicating zone system. The Inverter can modulate down to 25% of total system capacity and the blower ramps down accordingly as well.  
    A 4 ton system that can ramp down to 1 ton is amazing. The zone system operates the Inverter based on a PID loop that monitors supply air temperature and how many zones are calling. 
    However, as Mr. Butler has warned, the 1 ton may still be more capacity than a single zone requires. This is where planning & design really become important, and avoid creating numerous small zones. 
    This technology along with good planning/design, has the potential to drastically reduce reliance on a bypass and possibly eliminate it. 

  89. modulation and capacity
    modulation and capacity control are fine but what happens when zone branch is flowing a fraction of it’s capacity? A register on an inside wall won’t have the throw to reach the wall (or window) where the heat gain is occurring.

  90. All, 

    By the way, I want to clarify a comment I made yesterday, concerning efforts to educate Contractors on preferred zone system and bypass design. 
    I stated that EWC was the only zone Mfr. trying to educate Contractors and that is incorrect.  
    The AHRI zoning technology section, whose membership consists of many OEM and Zone Mfr’s, are aware of this issue. 
    I believe that each member is trying to educate their customer base on proper zone system & bypass design, including how to reduce reliance on the bypass, as the only means of managing airflow on a zone system. 

  91. Mr. Brown, 

    Mr. Brown, 
    Yes, inverter technology has come along way… 
    more and more manufacturer’s are getting into 
    it I think as more of a way to ‘lock’ a customer 
    into their way of doing things as from what 
    I have seen these are all proprietary systems.  
    They are proprietary because the equipment 
    communicates with the thermostat, sensor or 
    other controls within the system to know what 
    to deliver and where. The communication  
    between the controls and the equipment is 
    equipment specific… you can’t mix and  
    match equipment. There’s pros as well as cons  
    for this mentality to put it simple is to say: 
    It costs money.  
    Inverter zoned systems are more complex in that  
    if you have a sensor in room as opposed to a  
    thermostat you can’t adjust temperature from 
    a sensor. It is a read only control to send info 
    back to the thermostat.  
    This complexity doesn’t effect just the  
    homeowner but also the contractor. The  
    controls typically do provide error codes  
    to the contractor to give clues where problems  
    are likely to be so it’s not like it would  
    be an insurmountable problem to diagnose  
    such a system.  
    But if the thing breaks down on a Friday night 
    over a holiday weekend, that’s gonna leave a  
    mark somewhere.  
    You know as well as I do they break down at 
    the worst possible time… for you and me.  
    But hey, that’s what we wake up for right?  
    Trouble, Trouble, Trouble.  

  92. Inverter based systems indeed
    Inverter based systems indeed provide additional flexibility when designing a zoned system, although there are some important caveats: 
    * inverter systems always have a limited range. The 75% turn-down ratio for the Daikin ducted system is highly atypical. For example, some sizes of Carrier’s GreenSpeed heat pump can only turn down 33%, evenn though it’s marketed as having a 60% turn-down (only the 3-ton model comes close to that). Ditto for the Lennox XP/XC21. Caveat Emptor applies. 
    * every BTU the system is oversized relative the TRUE design load (which is nearly always less than what Manual J predicts) reduces the compressor’s dynamic range. In many cases, these systems end up with little if any effective range to handle less than ‘all-zones’ calls. 
    * to Ray’s point, cost, complexity and reliability are indeed issues with these new inverter drive systems.  
    Fortunately, there are less expensive and very effective ways to manage airflow without the use of either inverter technology or bypass. 
    @Eric, the issue you raise has to do with VAV duct sizing and is not specific to inverter technology. In any case, the more efficient the shell, the less important exit velocity (throw distance) becomes. In homes with inefficient shells, you can manage this problem in the design.

  93. @Eric If the system is
    @Eric If the system is designed to deliver to a certain number of registers all the time (not dampered) and that a portion of the capacity is sent to the zone calling for conditioning (the dampered runs) then the velocity at each operating terminal can be kept near constant, giving good throw.

  94. Ray & David,&nbsp
    Ray & David, 
    While it is true that cost and complexity is an issue with these Inverter drives, the cost will come down over time.  
    The EWC communicating zone system (UT3000) accepts standard 24v non-communicating thermostats, thus lowering the installation cost. 
    Because the zone system translates the 24v T-stat demands into digital operating protocol, some of the complexity is eliminated, which makes installation/troubleshooting easier. 
    Although, it may take a long time, just as ECM motors are replacing PSC motors, communicating HVAC systems will eventually replace legacy 24v systems.  

  95. John, thanks for your comment
    John, thanks for your comment. The conversation had migrated to capacity control and with that comes reduced airflow when not all zones are calling. The CFM will decrease as will the static pressure. The air volume could stay constant in a well designed duct system but I think the velocity will decrease

  96. Mr. Brown, 

    Mr. Brown, 
    Inverter systems will never come down that much 
    in price unless there is a better more efficient  
    solution. Especially since an inverter system is a 
    replacement of everything that makes up an HVAC 
    system up to and possibly including the duct  
    system. No piece meal type set ups.  
    A communicating system is one thing… most of 
    these quote un quote communicating systems  
    hitting the market are still backward compatible 
    but these aren’t inverter systems.  
    What separates inverter systems from just merely 
    communicating is how the proprietary control 
    is used for more than just temperature control  
    Inverter systems get into the realm of algorithms  
    that are run continuously to ramp up or down  
    the blower, the compressor, the electronic TXV 
    valve as well as learning behavior, not to mention 
    report trouble to the main controller, inform user 
    the need for service etc.  
    A regular communicating system is really  
    in my opinion nothing more than a way for  
    the manufacturer to sell a thermostat because 
    the contractor can cut the number of wires  
    needed to operate the system. This is way 
    different than that of an inverter system. For  
    one a regular communicating system is  
    typically backward compatible to non- 
    communicating thermostats… not all but  
    most. However, when using regular thermo- 
    stats you would need to have the proper  
    number of wires to control the equipment in 
    An inverter type system? nope. Entirely 
    different ball park. You must use the  
    proprietary controls to operate it.  
    For this reason it won’t work with  
    competing zoned methods. It’s all  
    proprietary controls only.  

  97. Eric, 

    You bring up a very good point, namely because  
    if you are going to get a complaint with a zone  
    system it will be when out door temps are at their 
    In my experience when I come across pre-installed 
    zone systems without bypass they increase the  
    size of the duct system as a way to deal with  
    excess air. Usually the systems I encounter with  
    these problems are not variable speed, but standard 
    two speed blower systems.  
    I suppose it would be quite possible to have the  
    problem you suggest and most likely a product  
    of poor design. But with that said there can be  
    a host of various trade offs with zone systems.  
    It’s important during design phase to minimize  
    these issues as best as possible.  
    The issue that I see is when heat load is at it’s  
    extreme, the duct system is over sized to relieve air 
    pressure when one zone is calling. So under heavy  
    heat loads when all zones are calling certain zones 
    don’t get the required air flow to decrease  
    temperature properly.  
    These kinds of issues are mostly design issues.  
    There is no miracle cure for design issues other  
    than to start over for the most part. It’s either that 
    or live with what you got and make the appropriate 
    adjustments to how you run and operate the zone  
    system in question.  
    Systems with bypass can have similar problems  
    or worse. Design issues are design issues… no  
    way around that other than to re-design.  
    I just got my July electric bill: $65.88 
    That is my total bill… AC cost? probably around  
    $25 for the month. I be breaking some records 
    this year! When I make the new video later this 
    year I will discuss the changes I made this year.  
    I looked over at my pet rock and I could have  
    sworn I saw him jumping up and down. LOL.  
    (He likes to be comfortable too ~ Stone Cold.) 

  98. Mr. Austin, 
    Mr. Austin, 
    An Inverter drive/system does not replace everything in the HVAC system, in particular the duct work. If only that were true.  
    A contractor can and will make the mistake of installing these systems without addressing duct-work issues.  
    In my opinion, the one single field factor that negatively affects any type of HVAC system’s efficiency, regardless of the level of sophistication, is undersized duct-work.  
    As you say, it comes down to design and detail. 
    Ray, of course these systems are complex…In order for OEM ‘s to produce higher SEER rated equipment, they must implement higher technology.  
    Digital operating protocols (standards) and Modulating systems are the result of government demand to increase equipment efficiency.  
    These systems operate proportionally and utilize demand usage algorithms & state of the art thermostats may utilize room temperature movement monitoring, in an attempt to match demand and avoid excess system capacity.  
    A 24v system cannot do that!…I believe Mr. Butler and I discussed the advantages of such a thermostat/system, not to long ago.  
    These communicating/modulating systems will dominate over time whether we like it or not. It is being mandated and forced upon the industry. 
    BTW…Have you found the P356 Explosive Bypass Modulator?  

  99. Quote from Mr. Brown:”An
    Quote from Mr. Brown:”An Inverter drive/system does not 
    replace everything in the HVAC system, in particular the  
    duct work. If only that were true.”  
    What I said was: “Especially since an inverter system is a  
    replacement of everything that makes up an HVAC  
    system up to and possibly including the duct  
    system. No piece meal type set ups.” 
    What does this mean? You can’t mix and match pieces 
    of equipment from your current system with pieces of 
    an inverter system to make up an HVAC system. 
    “Up to and including duct system.” What does this mean? 
    If you are replacing the WHOLE HVAC system- 
    furnace or air handler, Evaporator coil & condenser  
    how old is your HVAC system likely to be?  
    National average is 15 years. In my market what do  
    ducts consist of? Flex duct. Flex duct deteriorates  
    over time. To replace duct or not replace comes down 
    to a host of various factors.  
    An evaporator coil of an inverter system contains 
    an electronic metering device and will not work with 
    non-inverter type system. Sure you could remove the  
    electronic metering device, but that is counter  
    productive. Right? 
    An inverter air conditioning condenser requires 
    an electronic metering device, inverter drive  
    system controller, matching furnace etc.  
    At this point I really don’t think you understand 
    exactly what an inverter system is… even with  
    me posting links to show you.  

  100. Ray, 

    Apparently I hit a nerve, sorry. 
    I was referring only to the duct-work. 
    There are Inverter drives that operate with specially tuned TXV’s not EEV’s 
    There are communicating systems that operate on “Open” protocols (non-proprietary). 
    And yes, the Furnace/Air handler, Coil & Line set must match. But that is the way it should be!  
    There are enough problems out there with Contractors mixing equipment and components. 

  101. Ray, 

    Now before you start thinking that I am referring to you, I am not! 
    I am sure you are a great Contractor and obviously take pride in your work.  
    But you must know that you are the exception, not the rule. 
    This is the exact reason OEM’s have created proprietary systems. 

  102. Mr. Brown,  
    Mr. Brown,  
    No, you didn’t hit a nerve it just sounded like we weren’t  
    talking about the same thing. Different markets make  
    for tough comparisons… like flex duct versus sheet metal  
    ducts as an example. I know Mr. Butler is in Arizona and  
    I believe Mr. Proctor is in California… so there’s differences 
    Almost all “true” inverter systems use Electronic. I  
    suppose some could use a tuned TXV but if you’re gonna 
    spend that much money why wouldn’t you want an  
    electronic one? LOL.  
    Certainly the equipment should all match at all times, 
    but we contractors are often enticed to do things various  
    ways due to reduce budget constraints of the customer…  
    which is why there will always be a lower end with out the 
    communicating or inverter type design.  
    In a perfect world it would be great to only have to work 
    on matched complete systems. But alas, it’s not a  
    perfect world.  
    I’m skilled enough to make just about any brand work  
    with opposing brands, while there are limitations to this  
    madness I have done it with a decent amount of success,  
    but I try not to make a habit out of it. I don’t bash one  
    brand over another… they all break sooner or later. I just  
    look to pros and cons as to what one system offers over  

  103. We are planning to build a
    We are planning to build a one-story 2,500sf home. My intent is to create three independent zones. The system would be a VRF system in which I would utilize three separate ducted high or medium static pressure air handlers for each zone. The idea is that by using a separate air handler for each zone, the ductwork footprint will be smaller and therefore should perform much more efficient than using a single air handler with three zone damper system for the entire home. 
    Zone One – Main Living area – 1,300sf 
    A. Kitchen 
    B. Dining 
    C. Living 
    D. Office 
    E. ½ Bath 
    F. Laundry 
    G. Pantry 
    Zone Two – Master Bedroom One – 600sf 
    A. Bedroom 
    B. Bathroom 
    C. Closet 
    Zone Three – Master Bedroom Two – 600sf 
    A. Bedroom 
    B. Bathroom 
    C. Closet 
    The difficulties will be locating the air handlers so they can easily be serviced when needed. The easiest option is just locate the air handlers and ducts in the attic and spray foam the ductwork, I don’t like the idea of placement in the attic, but it seems every time I come up with some ideas for locating the air handler in the conditioned space, I run into to many problems with serviceability and ducts interfering with the design of the interior. 

  104. VRF systems are for the most
    VRF systems are for the most part ductless  
    mini splits… there are some ducted models  
    as well that incorporate various methods to  
    cool from centrally located cassette type air 
    handler with evaporator coil.  
    This is a zoned system and not a zoned  
    system in and of itself [at least not apart of  
    the discussion we have had thus far that I am 
    aware of]. The mini split market approaches 
    zoning by using complex refrigerant controls 
    and more equipment to pull it off.  
    The biggest problem is warranty periods are 
    rather short for these and majority of them  
    are foreign made. Parts are typically not  
    interchangeable between brands, meaning  
    that parts are brand dependent and can not be 
    intermixed with other brands. Due to the fast  
    change over of technology in this segment of  
    the HVAC market it is unlikely to find any  
    needed parts quickly. There is no distributor  
    that I know of in my area that stocks parts for  
    these and in many cases even ordering up a  
    new VRF system and components could take  
    up to two weeks depending on manufacturer  
    / distributor and brand used.  
    Additionally, many of these types of systems  
    are inverter driven systems that require a good  
    stable power source due to fragile electronics  
    within the system. (Power surges / brown outs  
    etc. will invoke havoc on them.) Servicing re- 
    quires special training and many HVAC  
    companies for that reason will not service them.  
    You will most likely be dependant on installing  
    contractor. If they go out of business for what- 
    ever reason you’re in the deep end. To put it  
    The zoning of this type of system comes from  
    complex refrigerant controls used to send re- 
    frigerant to the various indoor cassette type air  
    handler units in which conditioning of the air is  
    These kinds of systems are very efficient and  
    deal with zoning in a completely different way,  
    but as you can see from above there are pros  
    and cons to this line of thinking.  
    There is no “free lunch” when it comes to zon- 
    ing regardless of how you attempt to do it.  
    Good luck,  
    Ray Austin 

  105. As for long delivery time for
    As for long delivery time for parts, it should improve given a few more years and VRF parts in supply houses should improve.  
    I do hate the idea that most manufactures have priority protocols which lock you to their products. But if you look at high end American manufacturers they also lock you into priority systems. 
    As you know their are pro and cons to everything when it comes to home design from the mechanical, to the building envelope. 
    If the American companies can work out all of the problems associated with damper based zoning systems, I might consider a zone damper based HVAC system. 
    But for now I think a dedicated air handler for each zone is the best direction to get the most efficiency out of a HVAC system. 
    I am assuming the Air Handlers which is nothing more than a coil and fan should be problem free for the most part.

  106. Richard,  

    American manufacturer’s are producing more  
    and more of these types of systems, so certainly  
    part supply problems “could” improve. However, 
    if you look at warranty periods even for the  
    American manufacturer’s the story doesn’t change 
    It’s true that high ducted systems will lock you 
    into certain ways of doing things, but  
    warranty periods for this type of equipment  
    will blow a VRF system away. A good majority 
    come with replacement guarantees up to 10 years 
    but if this system is apart of a new home build 
    don’t expect to find that. New construction is a 
    different market… but if the warranty is there  
    for the retro-fit market it attests to how good  
    the equipment is. (replacement guarantees are 
    conditional so just because a unit breaks doesn’t 
    mean it will be replaced, but it’s still great  
    protection in the event you need it.) 
    There will always be problems with ducted 
    based zoning. Just as much as there will be  
    problems with any other kind of system. Up to 
    and including VRF systems. You have to  
    realize that VRF system will require a design 
    just as much as a regular ducted DX system.  
    You don’t escape design flaws by using a  
    different type of system… you just add more 
    complexity to it with far fewer options to  
    get service.  
    quote:”Air handlers for VRF are nothing more  
    than a fan, and coil should be problem free?”  
    Nope. The air handlers to these types of sys- 
    tems contain a fan, coil & electronics that  
    control the system. “the brains of the unit” 
    There are horror stories but you will have  
    to “dig” for them. Because products have 
    such short warranty periods it is very easy 
    for specific equipment models to drop off  
    the face of the earth.  
    Refrigerant leaks are common just after  
    the warranty period runs out, electronic  
    failures can happen after 1 good size  
    power surge. Nearby lightning strike etc. 
    If they don’t make the indoor air handler 
    any more and you can’t find one you  
    will be looking at buying a brand new  
    system -indoor and out because the old 
    will likely not work with the new one. 
    They will give reasons of “obsolete,  
    improvements to software or anything  
    else that looks good on paper.”  
    The refrigerant piping is different from  
    that of a regular HVAC unit so trying  
    to convert from VRF type system at that 
    point will be very costly and possibly 
    impossible due to duct run constraints.  
    I’ve looked into these systems and they 
    are infants…. way too soon to take the  
    plunge if you ask me. Nightmares!  
    A home builder is on the hook for 1 year 
    I don’t know of any home builder that 
    offers a warranty on “any” HVAC  
    system past the fist year period. That 
    spells big trouble for VRF systems that 
    are sold with new construction. 

  107. For those of you who might be
    For those of you who might be interested in even greater aspects of difficultly associated with VRF zone systems I thought I would share a glimpse of competing technology.  
    There are diagrams as well as video that show a “two pipe” set up versus a “three pipe” set up. 
    Mitsubishi claims to have the only “two pipe” 
    system for a VRF system. There is a video on that page also in the upper right corner of the page… (link to page down a few sentences.) 
    This design doesn’t really go that far other than to just give a small glimpse how designs of these systems can be easily flawed. Notice how they claim to be the best, but warranty terms or length of warranty for such systems are ironically absent… you betchya!  
    The website is here so you can take a look if you like…;
    So if you decide to go with Mitsubishi because  
    they claim they are the best with the “only two 
    pipe” VRF system available and you have nothing but trouble with it, then to switch to a  
    competing VRF system the piping design would have to be redone. No access to pipe design because you built the house around it?  
    Trouble, Trouble, Trouble!  
    In reality piping design would probably have to be redone anyway because different manufacturer’s of these system have different requirements, different refrigerant control boxes, different fitting requirements, different line sizing and total horizontal piping run length requirements, different vertical rise constraints and so on. Just another reason among many why most HVAC companies will stay far, far away from this nightmare… except maybe to give you a quote to rip it all out… LOL. (imagine falling over at the cost.)  
    The moral of this story… if you go with a VRF zone system you are stuck with a VRF system of the manufacturer you choose… unless of course you are made of money and can afford a complete gut job… depending on what access you have to run ducts and put in a regular ducted HVAC system.  
    An energy efficient system must be more than energy efficient. It must be reliable, it must offer decent warranty coverage of 10 years on equipment parts or better if possible. 5 year minimum on accessory parts and all parts must be readily accessible with little to no wait time to get system back on line.  
    If you settle for anything less, you will regret it. If I saved you from making a big mistake… wave at me. 🙂  
    Stay cool my friends… I know I will. You betchya! Petting my rock. (You have to read a ways back to get that one, I like inside jokes because it’s too hot outside!)  

  108. Thanks Ray for the
    Thanks Ray for the refrigerant line issues you brought to light. We still have about 2 years before we start the build, so things my change by than.

  109. Not sure if someone mentioned
    Not sure if someone mentioned this already, but your bypass is missing the damper. I recommend contacting an ultra zone company like EWC and have them explain that the bypass needs a damper. This is common knowledge.

  110. Jason: The
    Jason: The bypass duct in the photo above does have a damper. It’s hard to see here, but the arm and weight are hanging down just to the right of the red arrow.

  111. Mr. Bailes, 
    Mr. Bailes, 
    It’s hard to see this counter weight bypass damper, but if it is to the right of the red arrow, then it has been installed on a vertical duct run and that will never work! 
    I don’t know why Installers keep doing this. A counter weight bypass damper (which has a limited control range anyway) is designed for a level horizontal installation only. 
    An electronic (motorized) bypass damper must be used on a vertical bypass duct! In addition, any bypass duct MUST have a volume control hand damper in series (downstream) of the regulating bypass damper. 
    This allows the Installer to BALANCE the bypass duct and restrict the bypass volume to the desired value! Without the hand damper and proper balancing, the bypass duct will become the path of least resistance! 
    Installations like these, are why the bypass has such a bad reputation. 
    The link below will be helpful. 
    Best Regards. 

  112. John B.:
    John B.: The perspective may be fooling you here because that section of the bypass duct is horizontal. It goes straight back into the return plenum. The only vertical section on the bypass is the short piece that comes down out of the supply plenum. 
    Thanks for the info and the link, which I’ve made clickable here.

  113. Thanks for the clarification
    Thanks for the clarification but I still can’t see it. 
    I highly recommend an electronic bypass damper, regardless of the duct orientation. They are vastly superior to any mechanical bypass damper. 
    In any case, this bypass duct lacks a balancing damper and will therefore become the path of least resistance, when one or more zone dampers close down.  
    That condition will allow a higher airflow thru the bypass duct, than was intended, which will adversely affect the Delta P and the Delta T of your system.  
    No wonder you were disappointed with the zone system. 

  114. John B.:
    John B.: Just to clarify, the photo above is not my house. I used that photo simply to illustrate what a bypass duct looks like. My personal HVAC system certainly has problems, as I confessed yesterday, but bad zoning isn’t one of them.

  115. Understood…HVAC problems
    Understood…HVAC problems come in all flavors. 
    Thanks for the discussion. 

  116. High end equipment is only
    High end equipment is only made in  
    2 ton, 3 ton, 4 ton & 5 ton  
    (residential market.)  
    In some cases they may not even make 
    a 5 ton system and may require you 
    to twin the furnaces and put in  
    multiple systems to accomplish the  
    larger loads that a 5 ton system would 
    The sizing argument is fraught with  
    danger… and a Manual J is easily 
    skewed if you just make up or guess 
    at insulation levels.  
    Where is Superman with his x-ray  
    vision when you need him?  
    Manual J’s just as much as negating 
    a bypass doesn’t compensate for poor  
    Give a salesman a manual J and a ruler 
    and what do you think you’re gonna get? 
    That’s the skinny version of this 
    serious sizing problem….

  117. @Ray, although it doesn’t
    @Ray, although it doesn’t change the thrust of your commentary on VRF, just as a point of clarification, not all multi-split systems are considered VRF, at least not according to the nomenclature used by manufacturers. 
    In general, commercial multi-splits are considered VRF and residential multi-splits are not. 
    Some examples: Mitsubishi City-Multi line is VRF, whereas Mr. Slim MXZ in not. Likewise, LG Multi V is VRF but Multi-F is not. And the Fujitsu J series is VRF but their HFI multi-split line is not. (Multi-F and HFI are functionally similiar to MXZ.) Commercial VRF systems, sometimes used in high end residential, employ an active refrigerant management system. Some models can even heat and cool simultaneously by exchanging heat between zones (“heat recovery”).

  118. Mr. Butler, yes it’s true
    Mr. Butler, yes it’s true there are so many different ways of doing things on the mini split side it’s enough to make one’s head spin to try to keep it all straight. (eyes crossed) 
    VRF comes from “variable refrigerant flow”… In my opinion this is a short descript method of telling you the compressor is likely variable speed and from what I could tell when I initially looked at this stuff is that VRF is usually used in a mini split zoned system.  
    However, you will find differences from manufacturer to manufacturer in the mini split realm. In fact there is another flavor of the VRF type system called VRV. In my opinion VRV is more likely for a commercial type set up, although some large McMansions could deploy this kind of beast if they wanted to. I really don’t know specifically what makes one system a VRF a VRV or just a multi zoned mini split, do you? (Unless the manufacturer tells me what it is.) 
    When I was talking about VRF, it was to discuss the hurdles and cons of that particular set up as a whole and not necessarily include the multitude of ways someone could go via other methods of using mini-split type set ups.  
    If anything your point is important in regards to the complexity as well as the non-standard ways of doing things via the mini split method. Meaning that if you choose one type mini-split method over another you will most likely be stuck with that choice or spend more money each time you decide to diverge from one manufacturer’s way of doing things.  
    With that said… very good point to bring up David.  

  119. Ok I decided to do some
    Ok I decided to do some checking on  
    this VRF vs VRV subject… this may 
    shed some light. LOL. (who knew?) 
    What is the difference between VRF & VRV?  
    They are essentially the same thing, EXCEPT the term VRV is copyrighted by Daikin. for example a system by Daikin may be called a VRV, but a similar system made by Fujitsu is a VRF.  
    VRV = Variable Refrigerant Volume, VRF = Variable Refrigerant Flow.… VRF or Variable Refrigerant Flow is the technology being applied by manufacturer’s such as Samsung, Hitachi, Daikin, etc.  
    VRV is the trade name for Daikin’s VRF product lines. Hitachi uses Set Free, while Mitsubishi uses Mr. Slim. These are trade names for VRF the uses inverter compressors.  
    Samsung uses DVM, Midea uses MDV, while Trane uses TVR. These VRF technologies however use Digital Scroll Compressors which is a new technology in the market.…  

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