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

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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.


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John Nicholas
Aug 26 2011 - 7:36am

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

Allison Bailes
Aug 26 2011 - 7:50am

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.

Danny Gough
Aug 26 2011 - 7:54am

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. 

Allison Bailes
Aug 26 2011 - 7:59am

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.

Chris Cadwell
Aug 26 2011 - 9:29am

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.

Allison Bailes
Aug 26 2011 - 10:07am

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.

John Phillip Brown
Aug 26 2011 - 10:34am

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.

Allison Bailes
Aug 26 2011 - 11:39am

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.

John Phillip Brown
Aug 26 2011 - 12:17pm

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.

Allison Bailes
Aug 26 2011 - 12:28pm

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.

Larry Koehler, P.E.
Aug 26 2011 - 12:44pm

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.

Allison Bailes
Aug 26 2011 - 1:06pm

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.

Sam Young
Aug 26 2011 - 1:24pm

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!

Allison Bailes
Aug 26 2011 - 1:36pm

Sam: Are you talking about Rick Chitwood?

Sam Young
Aug 26 2011 - 1:38pm

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

Dan Kerr, P.E.
Aug 26 2011 - 2:25pm

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.

Larry Koehler, P.E.
Aug 26 2011 - 2:42pm

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...

Allison Bailes
Aug 26 2011 - 2:46pm

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

Allison Bailes
Aug 26 2011 - 2:54pm

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. ;~)

David Butler
Aug 26 2011 - 4:03pm

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.

John Phillip Brown
Aug 26 2011 - 4:13pm

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.

Allison Bailes
Aug 26 2011 - 6:40pm

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.

Donald B.
Aug 26 2011 - 10:25pm

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?

David Butler
Aug 27 2011 - 2:52am

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.

Allison Bailes
Aug 27 2011 - 9:48am

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.

dewey neese
Aug 29 2011 - 11:17am

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.

Allison Bailes
Aug 29 2011 - 11:33am

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!

David Butler
Aug 29 2011 - 12:53pm

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.

Mike Holscher
Aug 29 2011 - 2:33pm

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? 

Ted Kidd
Aug 29 2011 - 9:24pm

"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.  

Eric Schuhmann
Sep 1 2011 - 1:35pm

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

John Proctor P.E.
Sep 21 2011 - 11:18am

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.  

Oct 23 2012 - 1:37pm

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?

Oct 23 2012 - 1:43pm

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?

Feb 16 2014 - 9:04am

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.

Mar 4 2014 - 7:06pm

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!

David Butler
Mar 4 2014 - 11:39pm

@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.

John Phillip Brown
Mar 6 2014 - 12:29pm

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.

Mar 6 2014 - 4:51pm

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. 

David Butler
Mar 7 2014 - 2:32am

@Richard, are you referring to City-Multi?

Mar 7 2014 - 9:25am

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.

John Phillip Brown
Mar 7 2014 - 3:42pm

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.

David Butler
Mar 8 2014 - 2:41pm

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.

John Phillip Brown
Mar 10 2014 - 11:29am

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

T. Thornton
Jul 2 2014 - 2:15pm

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.

David Butler
Jul 3 2014 - 4:40pm

@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.

David Butler
Jul 3 2014 - 5:12pm

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.

John Proctor, P,E.
Jul 3 2014 - 7:44pm

@ 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.

John Phillip Brown
Jul 7 2014 - 11:49am

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. 

John Jones
Jul 10 2014 - 12:32am

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