Turn off the electrical power to the condenser unit at the outdoor shutoff. Either pull out a block or move a switch to the off position. If uncertain, turn off the power to the AC at the main electrical panel.

Vacuum grass clippings, leaves and other debris from the exterior fins with a soft brush attachment. Clear away all bushes, weeds and grass within 2 ft. of the condenser.

Realign bent or crushed fins with gentle pressure from a dinner knife. Don't insert the knife more than 1/2 in.

Unscrew the top grille. Lift out the fan and carefully set it aside without stressing the electrical wires. Pull out any leaves and wipe the interior surfaces clean with a damp cloth.

Spray the fins using moderate water pressure from a hose nozzle. Direct the spray from the inside out. Reinstall the fan.

Clean your outdoor unit on a day that’s at least 60 degrees F. That’s about the minimum temperature at which you can test your air conditioner to make sure it’s working. The condenser usually sits in an inconspicuous spot next to your house. You’ll see two copper tubes running to it, one bare and the other encased in a foam sleeve. If you have a heat pump, both tubes will be covered by foam sleeves.

Your primary job here is to clean the condenser fins, which are fine metallic blades that surround the unit. They get dirty because a central fan sucks air through them, pulling in dust, dead leaves, dead grass and the worst culprit— floating “cotton” from cottonwood trees and dandelions. The debris blocks the airflow and reduces the unit’s cooling ability.

Always begin by shutting off the electrical power to the unit. Normally you’ll find a shutoff nearby. It may be a switch in a box, a pull lever or a fuse block that you pull out (Photo 1). Look for the “on-off” markings.

Vacuum the fins clean with a soft brush (Photo 2); they’re fragile and easily bent or crushed. On many units you’ll have to unscrew and lift off a metal box to get at them. Check your owner’s manual for directions and lift off the box carefully to avoid bumping the fins. Occasionally you’ll find fins that have been bent. You can buy a special set of fin combs (from an appliance parts store) to straighten them. Minor straightening can be done with a blunt dinner knife (Photo 3). If large areas of fins are crushed, have a pro straighten them during a routine service call.

Then unscrew the fan to gain access to the interior of the condenser. You can’t completely remove it because its wiring is connected to the unit. Depending on how much play the wires give you, you might need a helper to hold it while you vacuum debris from the inside. (Sometimes mice like to over-winter there!)

After you hose off the fins (Photo 5), check the fan motor for lubrication ports. Most newer motors have sealed bearings (ours did) and can’t be lubricated. Check your owner’s manual to be sure. If you find ports, add five drops of electric motor oil (from hardware stores or appliance parts stores). Don’t use penetrating oil or all-purpose oil. They’re not designed for long-term lubrication and can actually harm the bearings.

If you have an old air conditioner, you might have a belt-driven compressor in the bottom of the unit. Look for lubrication ports on this as well. The compressors on newer air conditioners are completely enclosed and won’t need lubrication.

Turn the power back on, then set the house thermostat to "cool" so the compressor comes on. After 10 minutes, feel the insulated tube. It should feel cool. The uninsulated tube should feel warm.

In most cases, you can simply restore power to the outside unit and move inside to finish the maintenance. However, the compressors are surprisingly fragile and some require special start-up procedures under two conditions. (Others have built-in electronic controls that handle the start-up, but unless you know that yours has these controls, follow these procedures.)

1. If the power to your unit has been off for more than four hours:

• Move the switch from “cool” to “off” at your inside thermostat.
• Turn the power back on and let the unit sit for 24 hours. (The compressor has a heating element that warms the internal lubricant.)
• Switch the thermostat back to “cool.”

2. If you switched the unit off while the compressor was running:

• Wait at least five minutes before switching it back on. (The compressor needs to decompress before restarting.) With the air conditioner running, make sure it’s actually working by touching the coolant tubes (Photo 6). This is a crude test. Only a pro with proper instruments can tell if the coolant is at the level for peak efficiency. But keep a sharp eye out for dark drip marks on the bottom of the case and beneath the tube joints. This indicates an oil leak and a potential coolant leak as well. Call in a pro if you spot this problem. Don’t tighten a joint to try to stop a leak yourself. Overtightening can make the problem worse.

Turn off the power to the furnace at a nearby switch or at the main panel. Then pull out the furnace filter and check it for dirt buildup. Change it if necessary.

Open the blower compartment and vacuum up the dust. Check the motor for lubrication ports. If it has them, squeeze five drops of electric motor oil into each.

Pull off the plastic condensation drain tube and check it for algae growth. Clean it by pouring a bleach/ water solution (1:16 ratio) through the tube to flush the line. Or simply replace the tube.

Poke a pipe cleaner into the drain port and clean out any debris. Reinstall the drain tube and turn the power back on.

The evaporator usually sits in an inaccessible spot inside a metal duct downstream from the blower (Figure A). If you can get to it, gently vacuum its fins (from the blower side) with a soft brush as you did with the condenser. However, the best way to keep it clean is to keep the airstream from the blower clean. This means annually vacuuming out the blower compartment and changing the filter whenever it’s dirty (Photos 7 and 8).

Begin by turning off the power to the furnace or blower. Usually you’ll find a simple toggle switch nearby in a metal box (Photo 7); otherwise turn the power off at the main panel. If you have trouble opening the blower unit or finding the filter, check your owner’s manual for help. The manual will also list the filter type, but if it’s your first time, take the old one with you when buying a new one to make sure you get the right size. Be sure to keep the power to the blower off whenever you remove the filter. Otherwise you’ll blow dust into the evaporator fins.

The manual will also tell you where to find the oil ports on the blower, if it has any. The blower compartments on newer furnaces and heat pumps are so tight that you often can’t lubricate the blower without removing it. If that’s the case, have a pro do it during a routine maintenance checkup.

The evaporator fins dehumidify the air as they cool it, so you’ll find a tube to drain the condensation. The water collects in a pan and drains out the side (Figure A). Most tubes are flexible plastic and are easy to pull off and clean (Photos 9 and 10). But if they’re rigid plastic, you’ll probably have to unscrew or cut off with a saw to check. Reglue rigid tubes using a coupling, or replace them with flexible plastic tubes.