WINDOW and WALL UNITS

Time required: 30 minutes per unit.

Clean the outside. Unplug the unit. If it is portable, take it outside or where the mess you create won't do any damage. Wet a cloth with a household cleaner, such as Fantastik, and wipe the front and sides. Next, remove the front cover and clean it thoroughly. On some models, the cover is held in place with screws -- others have hidden tabs along the sides that pop free when pressed. Also, clean the plastic flaps on the front cover that direct airflow into the room.

Helpful: If the front cover or flaps are broken, check the manufacturer's Web site for replacement parts. You can buy replacements for many items, including control knobs and the expanding curtains that extend from the sides of window-mount air conditioners.

Clean the air filter if it's not too worn. The filter is a rectangular piece of foam or a fiber screen usually fitted on a plastic frame right behind the front cover. In some models, it slides out of a slot in the front cover. It may need cleaning once a month during the cooling season. Foam filter: Wash the filter under a faucet with warm water and gentle kitchen soap, such as Dawn. Fiber filter: Consult your owner's manual. You may be able to hold the filter under the faucet and let warm water strike the cleaner side. This forces dirt out of the pores. Let the filter air-dry.

Caution: To prevent mold and mildew, make sure the filter is completely dry before installing it. If the filter is torn or looks thin and worn down, it needs to be replaced. Check the owner's manual to find the right replacement filter. In some cases, you can use a generic filter that you cut to size. Available at home-improvement and appliance stores. Cost: Less than $10.

Clean the fins of the evaporator coils. Use a paintbrush or a vacuum with a soft-brush attachment to clean these thin, delicate strips of aluminum directly behind the filter. To avoid bending the fins, clean in a gentle motion in the same direction that the fins run. Bent fins block the flow of air into the unit, limiting its cooling ability and causing a whistling sound. Straighten any bent fins gently with the tip of a kitchen knife or a fin comb -- available at home-improvement and appliances stores for about $15. Make sure the thermostat sensor has not slipped from its bracket and is not touching the evaporator coils -- otherwise the temperature readings will be incorrect. The thermostat is a metallic two-inch bulb on the end of a thin copper tube.

Clear the condensation drain. This small drain, located below the evaporator coil fins at the bottom of the unit, channels water that condenses on the coils to a drip pan. Insert a straightened paper clip or wire hanger into the hole to ensure the drain is not clogged.

Caution: You can wipe down the back casing of the air conditioner with a cloth and household cleaner, but never remove the casing -- which contains the fan motor, electrical control box and compressor -- to clean inside.

Reinstall portable units. If you removed the air conditioner from the window, reinstall it, making sure the outside bottom portion of the casing is sloping down at least one-quarter inch below the windowsill. That's so the water runoff is channeled away from the home. Inspect the weather stripping between the unit and the window frame. The side curtains should fit snuggly against the window frame. Adjust or replace them as necessary. Fill gaps with weather stripping.

If there is foliage or other vegetation near the unit, trim it back to about two feet away from the unit.

Turn on the air conditioner, and listen for unusual noises. Any unusual humming may be caused by some sort of vibration. Turn off and unplug the unit -- and check for loose fasteners or screws that might be causing the noise. If the thermostat bulb is touching the coils, that could make a noise. A clicking noise may be the fan striking the unit's housing. Try shifting the unit or tilting it backward slightly. A gurgling noise, accompanied by water dripping down the front panel, indicates that the unit isn't draining properly. Make sure the cabinet's outer section is sloping downward out the window.

Check that cold air is blowing. If not, see if the thermostat is secure and the air-inlet flaps on the front cover are open. If the room is hot, give the unit at least 30 minutes to cool the space. If the problem persists, your machine could be leaking refrigerant or having other mechanical problems. Contact a home-appliance repair professional. Cost: $75 an hour and up.

Helpful resource: For information on energy efficiency, installation requirements and maintenance tips, go to the Web site of the Association of Home Appliance Manufacturers (AHAM) at www.cooloff.org.

CENTRAL AIR-CONDITIONING

Time required: 30 minutes, as well as 60-plus minutes for an annual checkup by a heating, ventilation and air-conditioning (HVAC) contractor.

Here's what you can do to keep your system operating at peak efficiency and limit the cost of the annual checkup...

Clean the condenser coil. It is located inside the large condensing unit that sits on a slab outside your house. Important: Make sure the circuit breaker that controls the power to the system is turned off. You may have to unscrew the protective metal casing around the unit to get to the coils, which are a larger version of the aluminum fins found in wall and window units. Vacuum the coils gently with a soft-bristle brush attachment. Clear away weeds, grass and other debris that block airflow through the coil. Straighten any bent fins the same way you would for a window unit.

Change the main air filter. It's usually located indoors in the main duct junction near your furnace. If you can't find it, have your HVAC contractor show you where it is and how to remove it. Consult your system manual for the correct replacement.

What your HVAC contractor should do: Verify proper operation... inspect for system design flaws... identify safety and code violations... replace hard-to-reach filters... recommend special filters for allergy sufferers... compare current and previous performance records.

To find an HVAC contractor: Ask friends for recommendations. You also can visit my organization's Web site, www.acca.org, and click on "Find a Contractor" under the "Consumer Info" tab. Average cost for annual service: $100 to $250.

Bottom Line/Personal interviewed Wes Davis, manager of technical services for Air Conditioning Contractors of America, a trade association in Arlington, Virginia. www.acca.org