Illustration by Jason Schneider
Ah, the sweet relief of a cool, dry house after a hot, sweaty afternoon of yard work. Like any mechanical system, AC needs a little TLC to run smoothly, including many tasks you can tackle on your own. We asked This Old House plumbing and heating expert Richard Trethewey to share his know-how for central air and window units.
Why and How to Change Filters
Dirty filters kill your AC's efficiency, so install a new one every month during the cooling season for central and window units (or clean them if you've got the washable type). Look for the filter's minimum efficiency reporting value, or MERV, which ranges from 1 to 12 for home AC units; the higher the number, the better filtration it provides (and the more energy needed to pull air through it, so balance air-quality concerns with energy costs).
Why and How to Fix Leaks
Ducts can lose up to 30 percent of airflow through leaks, and window AC units are notoriously tough to seal properly. Find leaks using the old "smoke trick": For window units, light a stick of incense and hold it where the unit and the window frame meet; for central AC, hold the stick near duct connections. If the smoke blows around, you've got leakage. For ductwork, use foil tape to seal small gaps and duct mastic for larger ones; for window AC units, stuff foam between
the device and the window frame, taping as needed.
Why and How to Use a Timer
There's no need to blast the AC at full tilt while you're at work. For central units, install a programmable thermostat that lets you set higher temps when you're gone and cooler temps when you're home. Newer window units have built-in timers and adjustable thermostats, or you can buy a timer at any home store for to ; just make sure it matches your device's voltage. Unless you're going on vacation, don't shut off the system; otherwise, the air compressor will need to work harder to cool your house later.
Why and How to Service Compressor/Condenser
A central AC system's air compressor and condenser are usually located outside the house, close to your foundation. It works best when there's about 24 inches of clear space in all directions, so get rid of nearby shrubs, tall grass, leaves, and hanging branches.
"Install condensers or window units on the north or east side of your house, or build a screen to shield them from the sun. Putting them in direct sunlight reduces their efficiency by as much as 10 percent."
—Richard Trethewey, Plumbing and Heating Expert
Why and How to Keep Unit Cool
Extend the life of your AC system by keeping blinds or shades down during the day; you can also install awnings to shield south-facing windows from intense sun. Consider running the AC in conjunction with floor or ceiling fans to circulate cooled air more effectively.