Clean Coils Keep It Cool

By Jeanne Huber
Special to The Washington Post
Thursday, August 24, 2006

Q Is cleaning the evaporator (not condenser) coils of a central air-conditioning unit a job the average do-it-yourselfer can tackle? If so, could you describe the process and potential pitfalls, please?

AYour question reveals that you understand some of the intricacies of maintaining an air-conditioning system. The condenser coil, which is outside, is fairly easy to clean, while the evaporator coil, which is inside, is trickier. Whether you can do it yourself depends partly on how handy you are and partly on what model you own.

Air conditioning is a multi-step process that involves two heat transfers. First, the equipment compresses a refrigerant, a material with properties that allow it to change between gas and liquid states at convenient temperatures. The refrigerant is a gas at this point, and as its pressure rises, it heats up. The supercharged gas flows outdoors, through the condenser coil, and transfers a lot of the heat to the outdoor air with the help of an array of aluminum fins. This cools the refrigerant enough that it condenses into a liquid -- thus, the name of this part of the system. Flowing back inside, the liquid refrigerant passes through a valve and expands, evaporating into a gas once again. Because it takes a lot of energy to change from liquid to gas, the area where this occurs, known as the evaporator coil, becomes very cold. Air blows over the coil and continues on to become the cool air that you feel coming out of the registers in your house.

Indoors or out, dirt on the coils acts as a blanket and keeps heat from transferring efficiently. Plus, a dirty evaporator coil can become covered with mildew -- a real problem since the air that circulates in your house passes directly over this part of the system.

Luckily, the condenser coil, which is easiest to clean, is also the part that tends to become dirty most frequently. Routine maintenance includes cleaning away fallen leaves or needles and protecting the box that houses the coil from grass clippings or dryer lint. If the housing does become dirty, you should wipe or vacuum it clean. Remove the cover and use a paint brush to clean debris off the aluminum fins, then rinse everything with a hose. Be careful not to bend the fins. If you do bend them, or find them already bent, you can realign them carefully with a $15 tool known as a fin comb or ask a heating and air-conditioning contractor to straighten the fins for you. Mike Brennan, an owner of Brennan's Heating and Air Conditioning Service in Arlington, cautions that if you don't do it right, you could cause more damage to the fins.

Cleaning the evaporator coil is trickier but doesn't need to be done as often. If the ducts in your house are sealed and the housing around the heating and air-conditioning components are well-built, virtually all of the air that passes over the evaporator coil should be passing through the filter of your air-conditioning system. In this case, simply changing the filter frequently -- each month, when you are running the air conditioner often -- may be all you need to do to keep the evaporator coil clean for years. If ducts aren't sealed or if the housing around the evaporator coil leaks air, you may need to clean the coil every year or two.

The easiest and best way to determine whether the evaporator coil needs cleaning is to pay a heating and air-conditioning contractor to check your system. Brennan charges $85 (or $134 to $154 for twice-a-year checks that also include heating systems). For that, his company oils motors if they need it; checks filters, refrigerant levels and electrical connections; and measures temperatures at different parts of the system. If the readings show the evaporator coil needs to be cleaned, a technician uses special cleaners to remove grime.

So the question is: Can you do this yourself? Some manufacturers design their equipment for easy access. Others make it so difficult that the coil needs to be removed and cleaned outdoors -- after the refrigerant is first drained, a job that should be tackled only by someone with the gear needed to make sure none of it spills.

If you don't want to spring for the annual checkup, ask yourself whether you're the type who can usually get pieces back together even when you don't have a diagram to fall back on. Most manufacturers recommend professional cleaning of condenser coils, so they don't give detailed instructions. If you pass this test, start by turning off power to the system. Open the cover to the evaporator coil, which is usually next to the fan. Look inside and see whether the coil and fins are easy to reach. If so, wipe them clean using warm water and maybe a little soap. If the fan motor is underneath, protect it with plastic so you don't drip water onto it. Also avoid using any cleaner that contains ammonia or other ingredients that would damage the aluminum fins. Professionals use solvent-based cleaners and sometimes resort to steam-cleaning -- after first removing the coil and taking it outside. If that is what's needed to get the coil clean, you'll wish you had started with a call to a pro.

"The 5 percent of homeowners who can fix everything, they will be fine doing this themselves," says Wes Davis, manager of technical services for the Air Conditioning Contractors of America.

For the rest of us, some professional help would be a good idea.

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