Hot weather really makes Pima County residents appreciate – or question – the performance of their air conditioners. If you think your AC is not keeping you cool and a bigger unit is needed, think again. It might be just the opposite and you are suffering from oversizing!
According to the U.S. Department of Energy, oversizing home ACs is a common and costly problem. New homes are being constructed to be more energy efficient, and owners of older homes are adding insulation, weather-stripping and better windows to reduce their energy use – requiring smaller, not larger, ACs. Getting a bigger unit than you need
“not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. … Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the ‘clammy’ feeling.”
A bigger unit may run less to cool your home, but in the same way that stop-and-go city driving uses more gas than highway driving, AC efficiency falls as the cooling cycle gets shorter. Plus, as with cars, too much on-and-off operation is hard on AC equipment. The most efficient and comfortable AC is one that runs all afternoon on the hottest days of the year.
Installing or replacing an AC requires a permit (unless it is a small window-type AC). Determining the right size of the unit requires the use of Manual J, “Residential Load Calculation,” published by the Air Conditioning Contractors of America, which considers such factors as how well your home is insulated, how much heat your windows let into your house and how much the duct work that distributes the air in your house leaks.
But you can estimate the size of the AC unit you may need by timing the cycles of your existing unit in hot weather. When the outside temperature is 105 or more, time how long your AC runs and then how long it is off before it starts running again. Divide the total into the minutes on to get the load. Do this three times and average the result.
Minutes on / Minutes on + Minutes off = Load
For example, if your AC runs for 20 minutes and then is off 10 minutes before it comes on again, your load is 20 / 20 + 10 = 0.67, or 67 percent of system capacity. Multiply the size of your unit by the percent of system capacity to find out how much cooling you’re really using. If your existing AC is a 3-ton unit, 3 tons x 0.67 = 2 tons of cooling. You might be better off with a 2-ton unit.
A trained professional AC contractor should assess all the Manual J factors to determine the size of the AC you need. But timing your cycle when it’s hot outside will give you a head start.
The Pima County Green Building Program helps homeowners and builders make homes energy-efficient, durable and healthy to live in. For more information, call (520) 740-6490 or visit www.pimaxpress.com/Green.