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Warning: Skipping This Step with New HVAC Systems Can Lead to Misery

Oversized Air Conditioner, Texas Style!

oversized air conditioner, Texas style!The typical new home gets a heating and air conditioning system that’s about two times too large. Look, I’ve  discussed oversized air conditioners many times before. Most HVAC contractors use a rule of thumb that says they should put in one ton of air conditioning capacity for each 500-600 square feet of conditioned floor area. New homes, though, typically need one ton for each 1000 or more square feet of floor area.

The typical new home gets a heating and air conditioning system that’s about two times too large. Look, I’ve  discussed oversized air conditioners many times before. Most HVAC contractors use a rule of thumb that says they should put in one ton of air conditioning capacity for each 500-600 square feet of conditioned floor area. New homes, though, typically need one ton for each 1000 or more square feet of floor area.

So the typical system’s too big, as you can clearly see in this photo I took in Texas on a recent trip. (And that’s just the AC for the garage!) Turns out, an oversized system is good, in a perverse way.

Why? Because the typical duct system is too small, has too many kinks, is made up of too much flaccid flex, and just generally can’t handle all the air from the oversized system. In many cases, though, it moves enough air to keep the house comfortable. It also provides some cushion, or elasticity, so that the HVAC contractor can get away with installing it that way and having it work OK, sort of.

Why this doesn’t fly with high-performance homes

In a high-performance home, the system is designed to be as close to the right size as possible. That means the ducts have to be designed and installed correctly, too. But the thing is, there’s not much buffer in this case. The cushion and elasticity are minimal here so that the system can do its job as well as it can. That means evening out temperature spikes and removing more moisture in humid climates.

What happens in a high-performance home if the design has an incorrect assumption in one place? Or if the flex duct wasn’t pulled as tight as it should have been? Or when the contractor doesn’t use rigid elbows? Or the builder uses wood grilles, with their much lower air flow?

I’ll tell you what happens. That system that was supposed to live happily on the good side of the razor’s edge slips onto the underperforming side, and it’s a bloody mess. How do I know? We do HVAC design here, and we’ve had a couple of cases where these things have happened.

Thou shalt commission thy ducts!

duct system commissioning hvac distress high performance homeThe thing is, the distress of unhappy homeowners can be avoided easily by spending a few hours commissioning the system to make sure not only that the equipment is working but also that the duct system is moving enough air.

If you’re an HVAC contractor and you’re not already commissioning every air conditioning system you install in a high-performance home, you’re going to face those callbacks. To avoid that, you may need to add some tools and techniques, but the heartache—and money—you save later on will make it worthwhile. Here’s what you need to do with every system:

  • Measure the temperature drop across the evaporator coil. It should be within the manufacturer’s spec’s, which is generally 15-20° F.
  • Measure the total external static pressure. This tells you if the blower’s able to do what it needs to do. It’s like blood pressure and shouldn’t be too high or too low.
  • Measure the total duct leakage. Even if the ducts are inside the building enclosure, you can have distribution—and comfort—problems if there’s a big leak somewhere, as we discovered in one project.
  • Measure the air flow from all the supply vents. In the photo above, I’m doing that with a powered flow hood (the FlowBlaster from the Energy Conservatory).
  • Measure the air flow on the return side. If the sum of the supplies doesn’t match the total return air flow, something’s wrong.
  • Check the mechanical ventilation system. Some configurations have the outside air being dumped into the return side of the air handler unit. If that’s the case, there should always be an electronic damper with a controller in the duct to the outside to prevent the air handler from from pulling in outside air during times when it’s not supposed to. If it’s simply a duct running from outside to the return plenum, the damper and controller are almost always there, but I’ve seen some ERVs installed without these necessary components. (More about this in a separate article later.)
  • Measure the mechanical system air flow. If the ventilation system is supposed to bring in 100 cubic feet per minute, 30 minutes per hour, how do you know it’s really doing that if you don’t measure it? And if you’re installing a balanced ventilation system (ERV or HRV), how do you know it’s balanced if you don’t measure?
  • Look for pressure imbalances in the house. Bedroom doors get closed at night. What happens to the air if those rooms have only supply vents? There should always be a way for air to get back to the system when doors are closed.

If the house is going for certification in the ENERGY STAR new homes program, all this stuff, and more, is required now that Version 3 is out. Not every high-performance home you come across is in a program like this, however, so it’s just a good policy to understand the nature of your projects and protect yourself from future heartaches by doing it right from the beginning.

Yeah, it’s more work than you may be used to doing as an HVAC contractor, but I can tell you one thing. It’s a heck of a lot better to do these things before the happy new owners move into the house, though. Before their smiles turn to frowns.


Related Articles

Why Don’t More HVAC Contractors Own Duct Leakage Testers?

7 Steps to Commissioning a New Home & HVAC System

How to Install Flex Duct Properly

This Post Has 16 Comments

  1. Great article, all so true;
    Great article, all so true; but I wish you stop saying that a high-performing house should have 1,000 sf/ton or “more”. An HP home with ≤1ACH50 and a right MJ should always be between 1,800-2,200 sf/ton or there is something wrong in the building enclosure, or the calculations. We see that our worse case scenarios with high leakage is around 1,100 sf/ton! Our issue is finding an HVAC contractors that wants to go there…

  2. I think you should simplify
    I think you should simplify your advice and first recommend that a high performance house, use a high performance air handler, i.e. an ECM variable speed blower. And add the needed caveats that this will not solve *all* problems, but it will solve some. 
    Excessive ESP: a Trane VS blower such as mine is designed and rated for decent air flow up to 0.9 inch ESP, well higher than most others.  
    Some variability in air flow. People might assume any airflow can be dialed in within range, any time. This is not the case, however one can make dozens of choices for air flow, and the blower will do what it takes to make this happen (within maybe 5%). 
    A main problem reported for oversizing, is humidity control. A VS blower addresses this by the ability to reduce air flow about 20% when a signal is sent from a humidistat or thermostat. 
    This is clearly not a whole solution, and anyone who does your full list of recommendations will see much better results (at a price). But it is one of the foundation stones of a well performing system.

  3. Great post- thanks Allison.
    Great post- thanks Allison.  
    In addition to all of the steps you mentioned, we add simultaneously measuring the Enthalpy at each supply outlet using fast response portable temperature and relative humidity loggers, as well as measuring the power consumption of the blower and outdoor units.  
    With this information, we are able to spot underperforming supply runs and ascertain how to fix or improve our installation technique (feedback loop). Typically the problem is either excessive length to destination, duct velocity lower than design, or insufficient insulation burial depth (ducts in unconditioned attic {gasp} deeply buried).  
    Of course the other component of good commissioning is the refrigerant charge and TXV adjustment. We throw away every non-adjustable TXV and bench swap it for an adjustable one on all installs, as even systems well within the HERS superheat test passing range still frequently leave 10% or more improvement potential “on the table” with excessive insurance (SH) that cannot be removed without an adjustable TXV.  
    In CA, our air conditioners are all at least twice as big as our homes need, due to all if the issues you mentioned. And our furnaces are all 4 times too large for the same reasons. All too often, manufacturers simply don’t make systems small enough, as is especially the case with virtually every furnace in mild CA. Getting rid of furnaces in CA and replacing them with heat pumps or combined hydronics (water furnace) is a good start toward eliminating the equivalent of your opening image on most homes.  

  4. hot humid climate perspective
    hot humid climate perspective here… 
    the problems here have evolved. now in addition to the 400 to 500 sq ft per ton hvac companies, some of the other companies have gotten to a point of kind of understanding why 3 tons for 2500 sq ft house is better than 5 tons.  
    kudos to them. however their solution is to still sell the 5 ton unit..but make it a more expensive two staged 5 ton system. so that it runs @ 3 ton capacity 90% of the time, and only ramps up to full capacity when home owner has an event with a lot of people & doors opening & know 4th of july crawfish boil time. 
    I’m not saying it doesn’t work…but just that the homeowner is paying not only the cost of 2 extra tons..but now a $1500 cost for two stage…or variable speed.  
    all so that the hvac company can:  
    1-still sell 5 ton unit 
    2-not get call out on 4th of july, memorial day, labor day etc. 
    3-make even more money off homeowner 
    as always the added cost is to the homeowner. always. 
    try & get a 3 ton on this house…and hvac companies walk away & simply won’t do it…thus the upgrade… upgrade to 5 ton… two stage. 
    I can count on one hand the hvac companies within 150 miles of me that have flow hoods. 
    while it is great to have more ammunition to fight these hvac battles…it is still a battle.  
    another ‘fix’ for oversized systems is to 
    add a whole house dehumidifier to a single stage 5 ton system on a 2500 sq ft house..lowers RH…but homeowner now has to buy dehumidifier…and pay the operating costs. 
    discharging 100 degree air into the supply plenum costs a good bit to overcome. glad to see the newer two piece dehumidifires that discharge to the ambient…but at a hefty cost. 
    great to read the articles…but it sure would make life easier here in the south if there was a standard for ventilation for existing homes…hint hint Joe..time to get it done. 
    as always, thanks Alison for 
    another good article.

  5. Allison – what problems have
    Allison – what problems have you encountered from too low static pressure? 
    Michael Chandler – if you’re looking for a low-cost way to estimate airflow at registers, use a vane-anemometer…or for a really low-cost way, use the garbage bag inflation method (
    Armando – ft2/ton has a lot to do with the ratio of glazing area to floor area. Among all high performance houses – bigger houses with relatively low glazing-to-floor area ratios are going to have high ft2/ton ratios. Smaller houses with higher glazing-to-floor area ratios are going to have lower ft2/ton ratios.

  6. Michael C. – John S.
    Michael C. – John S. mentioned the very affordable garbage bag method which Bruce Manclark demonstrated at a pre-conference session at ACI this year. The procedure in the link John attached works but it’s a little difficult to determine when the bag is completely full. Bruce added a step to his procedure where he monitors the pressure inside the garbage bag to determine when the bag is full. When the pressure spikes, bag is full. He utilized a twist-in collar instead of a clothes hanger, which makes it easier to attach/insert a pressure probe. 
    At the same session, Bruce mentioned that his favorite tool was one he constructed using a flow grid from TEC and cardboard. He claims it was very accurate and (slightly) more professional looking than the garbage bag method. The TrueFlow meters aren’t as cheap as a garbage bag, but certainly cheaper than a flow hood. 
    Mike M. – Kudos. Your continuous improvement process is fantastic. Out of curiosity, how long do you leave the data loggers in place? Is it a permanent part of the installation or do you remove them after a year? Do us all a favor and publish your learnings and commissioning best practices!  
    With that said, I wouldn’t necessarily include the learning that “last shuttle” means last shuttle. 🙂

  7. Jeffrey, 

    My IPad is now sprayed with milk, thanks to your message. And had it not been for those darn flip flops of yours, I really think we could’ve made it. 🙂 
    Great times- send me your email so we can keep in touch!  

  8. Michael C.
    Michael C.: As both John S. and Jeffrey S. mentioned already, the garbage bag method is your least expensive option and is surprisingly accurate if you do it right. For exhaust fans, use the exhaust fan flow meter from the Energy Conservatory. See my article on bath fans in ENERGY STAR homes for details: This Little Hitch Snares ENERGY STAR Homes
    Armando: We can certainly debate what the threshold should be, but in my book, 1000 sf/ton is where you can start to think about a home being high performing. It certainly doesn’t guarantee it, and we can easily do better.  
    M. Johnson: Absolutely. Using more expensive equipment can be better, but it doesn’t absolve you from having to commission the ducts in my book. An ECM blower attached to a restrictive duct system will use more energy than a PSC motor. 
    Mike M.: Great advice. I wasn’t aware of the TXV issue, so thanks. 
    Debbie: HVAC companies selling a 5 ton, 2 stage AC for a house that really needs only 3 tons are copping out at best, upselling just because they can at worst. 
    John S.: My point there was that you can get a really low sp by removing the ducts. Also, if it’s too low, the throw might be really poor. I know David Butler says that throw isn’t as important in high performance homes, but it’s still somewhat important, especially if all the supplies and returns are in the same plane (floor or ceiling). 
    Jeffrey S.: Uh huh. I told you guys when I left that you might get stranded. Good thing you didn’t have to walk back to the hotel, like poor Danko did at 4 in the morning. 

  9. Allison,  

    I would argue that throw is a function of velocity leaving the register along with register design/shape and location….and has nothing to do with external static pressure. 
    High ESP does not necessarily equal long throws. Low ESP does not necessarily equal short throws.  
    Theoretically you could have extremely low external static pressure (by using aerodynamic fittings, low velocity/large-ish ducts and short runs) and use the register to increase the velocity of the airflow as it leaves the system….giving you the desired throw.

  10. John S.:
    John S.: You’re absolutely right. Not sure what was going through my brain when I wrote that, but I really did learn the continuity equation (A1*v1 = A2*v2) in intro physics, so I should’ve known better. I was wrong. 
    For those who want to know a little more about the continuity equation for fluids, here you go: 
    Fluid Flow and the Continuity Equation

  11. This is the perfect example
    This is the perfect example of book smart and real world smart. Most HVAC contractors fortunately excel at the latter while only some energy auditors excel at the first.  
    A: You assume A/C’s always run at 100% efficiency. Real world: Homeowners don’t replace filters when needed, may want their house to be 70 when it’s 110 outside and have every right to, and have there A/C serviced only when there is a problem.  
    B: People that spend hundreds of thousands on a house put comfort above efficiency. I know, big shocker. 
    C: If you have 10 people over for a party, what are you supposed to do when your A/C isn’t designed for that? Let them all sweat it out in the name of KWH’s?  
    D: Every occupant is different in the real world, but when you are a book smart energy guru that isn’t the case, you have a “standard” occupant.  
    E: Many HVAC contractors have serviced and installed thousands of units, believe it or not, they know what is best, yet are looked down upon by todays new wave of “energy geniuses” that have never had to worry about customer being comfortable and your businesses name being tied to that happy customers.  
    F: A high performance house is like a high performance car, makes no difference if the driver sucks. 
    Welcome to the real world.

  12. Greg:
    Greg: Based on your comment, it looks like maybe you read the first couple of paragraphs only because you haven’t addressed the main point of the article at all. If you can calm down a bit, stop assuming you know what I think, and read the whole article, you may be surprised to find out how much we actually agree.

  13. Greg: I
    Greg: I just checked out your website and, although your comment makes it sound like you’re ‘real world smart’ HVAC contractor, you seem to be one of those ‘energy geniuses’ who does energy audits.  
    By they way, since you’re in Austin, maybe you’d be interested in this other recent article I wrote about air conditioning in your great city: 
    Are You Making These Mistakes with Your Garage? 
    Also, you might be interested in attending ACCA’s Building Performance Forum. I’m giving the opening keynote speech.

  14. Perhaps some “book smart
    Perhaps some “book smart” people have developed a measure of disconnect from the “real world smart” realm, but not the ones I know. 
    So let’s look at Greg’s alphabet bullet list: 
    A) If the system was designed from the beginning to maintain a house at 70 degrees indoors with 100 degree outdoor ambients, it’s not oversized for those particular days. The rest of the time, it is, as is every other residential HVAC system operating under less than design conditions.  
    Poor maintenance habits are no excuse to oversize. In the case of neglected filters, all you have is more surface area on the evap that can freeze up. 
    B) People who spend hundreds of thousands of dollars on a house…how many hundred thousands? 100? They expect comfort at that level, but also energy efficiency. 1,000 hundred? Depends. Some with that much cash don’t care, and build a monstrocity with high glazing to square foot ratios. Others do care, because they became wealthy by not being foolish in how money is spent. 
    C) So, the homeowner pays for and operates equipment designed for events that happen – what – how often? And the rest of the time it’s oversized unless it’s a design day. 
    D) “Standard” HVAC design conditions are based on a consensus of comfort developed by ASHRAE and ACCA. If your clients define comfort cooler or warmer than that, nothing is stopping you from designing toward that end.  
    E) Any “energy guru” worth his or her salt knows that comfort and energy consumption are coincident. In fact, they may know that many homeowners pay too much for comfort, due to poorly designed HVAC systems (and they may also know how to design AND install it properly). 
    F) Poor design and poor installation render poor results. That’s real world, also.

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