Comments on: Why an Oversized Air Conditioner Is a Bad Idea

By: Mark Miller

Mark Miller — Wed, 24 Apr 2013 21:53:26 +0000

Great article. And make sure
Great article. And make sure you keep that evaporator coil clean.

By: Brian Crowther

Brian Crowther — Fri, 05 Apr 2013 00:20:54 +0000

Just a comment, but even in
Just a comment, but even in desert-like climates in the southwestern US, condensation can be an issue on compressors and refrigerant lines, so make certain yours are all insulated properly. Even very low humidity in the summer can result in mold growth, especially if your A/C unit is hit by sprinklers, which many are.

By: Worlock Air Conditioning

Worlock Air Conditioning — Tue, 28 Aug 2012 21:23:18 +0000

great blog, thank you and
great blog, thank you and keep it up!

By: John Proctor

John Proctor — Sun, 26 Aug 2012 08:14:15 +0000

@ David Butler Lets start
@ David Butler Lets start with what we seem to agree on. Attics are a bad place for ducts, particularly if they are poorly insulated and long (large surface area).

The cycling thought cuts both ways. The smaller unit's compressor-on cycle is longer than for a larger unit. During that longer on cycle the temperature difference between the supply air and the attic is at its greatest, thus the highest heat loss per minute. The off-cycle in both cases is controlled by the thermostat deadband and the heat gain of the structure, therefore it is darn near the same length of time. During the off cycle, the heat gain is limited by the volume of air in the duct. There are less total cycles with the smaller unit, there is more time spent at the higher heat gain. Making the smaller unit have a higher conduction heat gain than the larger unit -- (all else being equal eg. duct surface area).

By: David Butler

David Butler — Sat, 25 Aug 2012 12:31:57 +0000

@Bill: I always discuss
@Bill: I always discuss temperature settings with clients during the design phase. I recall one lady who said she sets her stat at 68 in summer. Digging further, I learned her 100-year old home had single-pane glass and little insulation. I asked her to buy a thermometer to monitor the areas where she spends most of her time (being careful to avoid lamps, sun and suppy registers). Turns out the e-m stat was off 2 degrees and there was an additional 4 degree swing between stat location and the living areas. After moving into her new home, the client happily reports being very comfortable at 75F. I don't think many people would be comfortable at 68F.

By: David Butler

David Butler — Sat, 25 Aug 2012 12:26:00 +0000

@John P: we can't always win
@John P: we can't always win the battle to keep hvac out of the attic. Don't you agree that cycle losses in this case are significantly greater, especially in hot climates? When attic is scorching hot, insulation can only do so much to keep the air handler and duct components from getting warm between cycles. At the start of each cycle, the system dumps hot air into the house, and all the heat absorbed by the metal components eventually transfers into the return and supply air. The larger the system, the more times this happens over the course of a summer.

@Allison: as you noted, with heat pumps there's an offsetting benefit to sizing a bit larger. But it's important to qualify that. Energy Star allows up to 25% oversize based on cooling load in certain climate zones, and I sometimes go even further. But I don't think anyone would argue that a system that's double the design load or worse is a non-issue. Unfortunately, this is what often happens in high performance homes when the contractor doesn't have any idea what the actual design load is.

By: Bob

Bob — Sat, 25 Aug 2012 06:08:27 +0000

We're going through A/C
We're going through A/C replacement at our house. Downsizing from a 3 ton to a 2 ton in a 1600sqft Oklahoma home. 800sqft per ton, but expect power bills and humidity control to improve.

By: Harvey Sachs

Harvey Sachs — Sat, 25 Aug 2012 04:03:05 +0000

Region matters a lot (both
Region matters a lot (both peak temperature and dehumidification need). Oversized single-speed very bad in SE, but oversized 2-speed with variable speed blower might be wonderful there - huge evaporator for mild conditions (high CFM/ton, too), but ability to increase latent work with high compressor/low fan. The big Q. is how to get consumers to value professional design, selection, and installation, to differentiate the "product" as including important services.

By: pj

pj — Fri, 24 Aug 2012 21:00:59 +0000

Thanks Alison, <
Thanks Alison,

On high volume, production RNC, we never know what direction a house will face. There are no North arrows on the plans, therefore we assume worst case to CYA. All houses of that design, get the same AC, same duct system, and same heater. Builders won’t even let us downsize AC’s if we know the orientation as two homeowners would start a fight over why they had the same house but different size units. In southern climates we have to oversize heaters to get the CFM for the ACs. So if are Manual J’s are perfect, the AC system is oversized at least 75% of the time, remembering that even the “properly sized units” were oversized 99% of the time, our duct systems are wrong 75% of the time, and our Furnace is oversized 100% of the time.

On replacements, Thomas says it best. Guessing at unknowns to make loads come out to what you want is no different that just guessing the size. To bad the focus is on a load that may happen for only seconds a year, rather than the constantly changing nature of reality. By taking the time variable out of the model in order to dumb down the math to a 3rd grade level, we have lost focus on both comfort and efficiency. Our most comfortable and efficient systems are those that modulate capacity to constantly changing conditions, not simply turning off and on at full tilt.

But the old load calc stick can be fun to watch, and listening to all the guys tell you their loads are right is priceless.

pj

By: John Proctor

John Proctor — Fri, 24 Aug 2012 20:35:25 +0000

@ Thomas B. The 20 degree F
@ Thomas B. The 20 degree F temperature drop is an incorrect rule of thumb. The temperature drop changes all the time depending on the indoor and outdoor conditions. More importantly, the airflow is what needs to be done well AND MEASURED. The target temperature split is a little better than the 20 degree rule of thumb, but measuring the airflow is the right way to do it.

See the following California Energy Commission web info on measuring methods as well as the less effective, but useful temperature split targets. http://www.energy.ca.gov/2008publications/CEC-400-2008-004/CEC-400-2008-004-CMF.PDF Look at Table RA3.2-3 and Section 3.3. for technical information.