Making Sure Your HVAC System Is Properly Sized

Part I of A Lot More Parts

If only people put as much thought into choosing a heating and cooling system as they did their kitchen countertops, our homes would be much more comfortable, energy efficient, mold-free places to live.  Alas, it is barely even on the radar.

It was on my radar. As a writer and marketing consultant for the HVAC industry, choosing an HVAC system for the Greenspiration Home was one of the first (and most expensive) decisions I made.   Our geothermal heating and cooling system was the “centerpiece” of our green home. So you can only imagine how upset I was to discover during our first cooling season that this unit had been grossly oversized, creating unsuitably humid indoor conditions inside my brand new home.

We have since had this problem resolved – but it wasn’t a pleasant journey. This was a big “oops” and no one was particularly excited about taking responsibility for it. Suffice to say I’m happy that I recognized the problem and was finally able to get it resolved.  Most homeowners, I dare say, would not be so lucky.

Why Bigger Isn’t Better
Oversized heating and cooling systems is not a problem exclusive to geothermal systems – not by a long shot.  The Air Conditioning Contractors of America (ACCA) estimates the average HVAC system in the U.S. to be 1 ½ to 2 times oversized.

So why is that a bad thing? There are actually several reasons, but the worst is the lack of dehumidification. If a cooling system is oversized, it runs for shorter periods of time because it so quickly satisfies the thermostatic set point; when a system “short cycles” like this, it doesn’t have time to pull the moisture out of the air.  So the temperature is right, but the air inside the home still feels muggy or clammy. (Stick with me, here. This is important—and it isn’t rocket science.)

Think about a glass of ice water on a hot day. The longer it sits in the warm air, the more condensation that forms on the glass.  In a sense, the cold glass is dehumidifying the warm air around it. The same thing happens with a cooling system—only instead of a glass filled with a cold beverage, a cooling system has coils that are filled with a cold fluid or refrigerant. If the system doesn’t run long enough, then the coils don’t get cold enough to pull much moisture out of the air.

Of all of the lessons I learned building this fabulously green home, what I learned about HVAC sizing is the most important.  I look forward to sharing this information. For now, I leave you with a few practical pieces of advice.

  1. Purchase a couple of humidistats for your home and check them regularly.  If, during the cooling season, you are experiencing sustained indoor humidity over 60% then you have a moisture problem.  It may or may not be the result of an oversized HVAC system, but the cause should be determined. Otherwise you leave your house at increased risk of mold.
  2. When it comes to installing an HVAC system in a new home or replacing one on an existing home, insist that the contractor perform (and provide you a copy of) what is known as a Manual J Load Calculation.  This calculation takes into account the unique heat loss characteristics of your home.  And no matter what a contractor might tell you, an HVAC system should never, ever be sized based on square footage alone.
  3. Consider having a 3rd Party weigh in on the sizing of the HVAC system for a new (perhaps even existing) home.  By 3rd party, I mean a HERS rater, better known as an Energy Star rater or even an engineer.  They can run their own Manual J calculation, which you can use to compare with your contractor’s calculation.  Yes, you will pay a fee.  Yes, it’s worth it.
  4. If the contractor and the HERS rater disagree on the size of the equipment, keep these two things in mind.  First, the HERS rater doesn’t make any extra money when you install a bigger system; the contractor does.  Second, in all likelihood the HERS rater has training in building science that the contractor does not.  This makes him more qualified to evaluate the heat loss and heat gain characteristics of your home.


Chew on that for a while.  I’ll be revisiting this topic in future blogs.

4 Responses »

  1. We have determined that our geothermal system is grossly oversized. Now what? Obviously, our contractor doesn’t want to replace the system and we’re hesitant to litigate because of attorneys’ fees.

  2. Thank you for drawing my attention to this earlier blog post about your geothermal system. After reading your April 6, 2013 post, I just had to know how this all started.

    In terms of operating cost, short cycling of oversized geothermal systems can be just as bad as undersized systems that run all the time. You’re exactly right – proper design is the key.

    I’m wondering if a smaller heat pump with a lower capacity and lower fan speed would do the trick? (I guess it depends if the ductwork is grossly oversized too.) Do you have any other posts about how the problem was resolved?


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