Oversize air-conditioning systems are the norm, not the exception. It’s easier to pick a huge system based on erroneous rules of thumb than to spend time designing a more suitable but smaller system. Oversize systems have the added problem of masking many
of the problems I’ve discussed here. Poor insulation, duct leaks, and more can be covered up by blasting twice as much cold air through the ductwork as would be necessary if things were done correctly.
If you double the size of the AC unit, you can lose 50% of the performance and still provide enough comfort so that the homeowner won’t call you back. But a behemoth AC unit short-cycles (turns on and off too
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Oversize HVAC Is Overkill |
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hen it comes to air-conditioning units, oversize air handlers waste energy, burn out faster, and leave the house cold and clammy. Unfortunately, many HVAC contractors still rely on rules of thumb to determine system size. The best way to get the right-size HVAC unit is to model your home’s energy features with one of the many software programs available (such as the one at www. hvaccomputer. com; $49 for homeowner version). To calculate whether an existing AC unit is too big, measure the number of minutes per hour that the AC unit runs on the hottest afternoons in the summer. Then divide 60 by the number of minutes to determine the amount that the unit is oversize. For example, 60 f 30 = 2x oversize; 60 f 20 = 3x oversize. |
quickly), which hurts its energy efficiency, degrades its ability to dehumidify the air, and shortens its life. A larger unit is also noisier and costs more to install (both system and ducts). The solution is simple: Pay for the load calculations, and size the unit correctly. In fact, according to the Air Conditioning Contractors of America (ACCA), it’s often better to undersize an AC system a little bit.
Water Heaters and Windows Are the Next Savings Opportunities
Water heaters store hot water all day long. They keep it hot on the off chance that you’ll need it. Tankless, or on-demand, water heaters, on the other hand, convert cold water into hot water when you turn on the tap.
You’ll notice that windows aren’t on this list. Only after you correct all the things I’ve mentioned will your windows start to look bad. If you’re building a home, upgrading the windows at the design stage to at least Energy Star (preferably beyond) is a lot less expensive than buying substandard windows now and replacing them later. Even so, windows are not usually the first place to start looking for big savings, because the other nosebleeds are running hard.
Bruce Harley is technical director of Conservation Services Group (www. csgrp. com) in Westborough, Mass.
