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New Home Builders Under-sizing A/C units


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I am finding more and more builders are under sizing their Air Conditioning units on new construction. Their view is that new construction is more efficient and Air Conditioning units do not need to be as large, even when they are installing only 10 seer units. During sales pitch one 2000 sq ft home buyer was told he was getting a 4 ton unit, only to find out from me that a 3 ton 10 seer was installed. Builders seem to be using an 80 - 90% rule. Deeming it acceptable for home owners to endure a lack of adequate Air Conditioning up to 20% of the year. Something like 73 day a year, it is acceptable for the unit to run non-stop. When the homeowner calls the builder out, they send a HVAC tech out to say that the unit is working to capacity. Or with the law of averages, some homeowners have no one home during the hottest hours of the day and only notice it on weekends. To me this is nothing more than a bate and switch scam. IMHO. Builders gear up to handle the calls for those days of the 1st year that are over 90 degrees, then they wash their hands of it.

I am documenting on my reports a picture of the AC faceplate and Air Conditioned sq. footage, verbalizing my opinion that the Air Conditioning capacity is or may be marginal. I tell he client how the differential temperature my not tell the whole story depending on the day, and that over time it may get worse. Should I be doing that? What are you guys doing on this? Any comments appreciated.

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I would be interested if the installing HVAC contractor is doing any heat load calcs or doing "rule of thumb" engineering.

New construction materials & insulation obviously go a long way in keeping energy demand down ala smaller HVAC units.

An over-sized air conditioner, IMO, is worse than one that is undersized. You might maintain a set temperature but the air conditioner never runs long enough to do an adequate job of humidity reduction (cold clammy conditions), not to mention short cycling.

If you state an opinion, on new construction, you should have your ducks in a row besides a flat "square footage / (X)CMF per square foot = tonnage.

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My thinking is like Terry's. It's my understanding that the unit should be sized to run continuously on the hottest degree days and still maintain an acceptable indoor air temperature. That should get the humidity out of the air as Terry says.

Also a unit running continuously is more energy efficient than one cycling on and off.

That's my contribution to inspector folklore.

Another thing - my contract says that I don't do design calculations. So I just report the size and let the buyer figure it out. I might say some verbally if it looks way off, but I don't write it down.

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Hi Chuck,

You'll find KB Homes putting a 5 ton unit in their 2 story homes up to 4200 sf. They swear that it's more than enough. My common reply is "Perhaps it is if we were in Michigan, but not South Texas"

What I normally do is tell the Client that the unit will not likely cool adequately to keep them comfortable, especially if both upstairs and downstairs zones are going at once (I've actually documented this several times on 1 Year Warranty Inspections). I also state in my report that the system needs proper load calculations performed by a qualified and licensed HVAC contractor. If the system is really bad, I just put in the report that the system needs evaluated by a Mechanical Engineer who specializes in design and repairs of HVAC systems.

So far, the few times my Clients have actually had an independent HVAC contractor out to do calculations, the builders HVAC team have been way off.

I generally recommend either Central City Air or Kingwood Air. Either of those companies are really good, especially David @ CCA.

By the way, the new TRCC Performance Guidelines state that the system is suppose to keep the inside temperature at least 15 degrees cooler than the outside air temperature. Pretty lame but I've seen times where I could actually use this against the builder.

Good Luck,

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  • 5 weeks later...

Don't always blame the HVAC contractor for lack of capacity. The good ones size the unit with the building specs that the general contractor gives them. Sizing according to Air Conditioning Contractor's of America manual J heat loss/gain calculation methods. Now if the general contractor puts it in 8 inches of insulation in the attic [:-censore instead of the specified 12 inches and the HVAC contractor sizes the a/c based on that 12 inches. Well..... you know what is going to happen when the outdoor temp reaches or exceeds design temp. The indoor temp may even rise.

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