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With AC, bigger may not be better

(ran HC HS HL)

Question: I'm sure our air conditioner won't make it through another summer. It didn't work that great last summer, and I have been putting off getting another one.

I would like to get a big unit when we buy one. A local contractor told me that wasn't a good idea, and he has general guidelines that determine the right-size unit. Will this make a difference?

Answer: I think you're both wrong.

Your idea for a large air conditioner might end up costing too much money to buy the unit, too much money to operate it, and then it probably won't adequately dehumidify your house because it will more quickly reach your desired temperature but in the process not run long enough to remove the humidity. Bigger is not better when it comes to choosing an air conditioner. Smaller isn't always good, either.

Your contractor is wrong to rely on a rule-of-thumb to estimate the right-sized unit for your home. Many builders figure out a rough estimate of the size unit they thinkyou need, then put in a little bigger unit to make sure that they figured properly. Because today's houses are a lot more energy-efficient than homes were just a few years ago, these guidelines are obsolete, and contractors need to follow detailed sizing guidelines from professional organizations to get the correct size.

One thing is clear: If you hire a contractor who calculates the numbers and properly sizes your air conditioner, you can save hundreds of dollars on the cost of buying the system, and the resulting better air conditioner run-time will give you better humidity control.

Be sure to factor in your lifestyle. If you do a lot of entertaining in the hot weather and keep the house open for large parties, a unit that is sized properly for your family needs may not be adequate for these special needs.

Your alternative would be to pre-cool the house and keep the thermostat set very low, defeating other attempts at energy efficiency.

Ken Sheinkopf is director of development for the Florida Solar Energy Center. Write to him at the Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922.