Part of the continuing myth-busting series.
Energy Myth: For Air-conditioning, a larger unit is better.
Reality: Bigger simply is NOT better. In fact, too big is much worse than the “right size”, and often worse than a little bit too small would be.
Unfortunately, because it allows them to take short cuts on determining the right size, and not worry about duct leakage or poor insulation, many contractors fudge it, and install a ton or more of extra capacity. Don’t fall into this trap. The benefits of a properly sized air conditioner are huge.
A sad thing is that an oversized A/C costs you on the front end and throughout its life. A bigger unit costs more. And thus you lose right out of the gate.
Oversized units “short-cycle”. That is they turn off quickly. Air conditioners don’t gain their full efficiency for several minutes after start up. If you’re A/C shuts off before that, it’s not operating at peak efficiency, and you can pay up to 10-20% more.
But it’s worse than that. Air-conditioners remove less humidity from the air at the beginning of their cycle. And longer run times help pull more moisture from the air. So an oversized unit, which runs less, will leave a house feeling clammy even as the air temperature is dropped.
Bigger capacity also means a bigger fan (assuming the contractor even looked at air-flow). And all else equal, a high air-flow means more noise—really high air-flow can create downright annoying noise issues. And the short run times don’t allow the air time to mix—you get blasts of cold air.
A too-big A/C costs more up front, costs more to run, won’t remove humidity as well, can be noisier, and less comfortable.
A right-sized A/C costs less than an oversized unit, is less expensive to run, does a better job removing humidity, can be quieter, and with longer run times does a better job mixing the air and delivering consistent comfort.
Don’t let your contractor talk you into something bigger than you need! And ask to see the sizing calculations, generally the “Manual J” sizing. [Even better, ask how you can tighten ducts, increase insulation, and reduce heat gains, to get an even smaller unit.]