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Central Air Conditioner Repair

Detailed instructions on how to diagnose and solve the most common central air conditioner problems.

In this article:

Air Conditioner Does Not Cool
Air Conditioner Does Not Turn On
Room Temperatures Experience Major Swings
Air Handler Makes Noises
Water Pools Next to Air Conditioner
Air Ducts Are Noisy

Air conditioning not cooling? Here is how to troubleshoot and repair typical central air conditioner problems.

Although some central air conditioner repairs must be handled by a qualified air-conditioning repairperson, you can handle simple repairs and maintenance yourself. Following, you’ll find DIY guidance based on the symptoms.

A central air conditioner that runs but doesn’t cool may just need to be cleaned. Plan to do this on a relatively warm day. The following are basic guidelines, but always refer to your owner’s manual.

Air Conditioner Does Not Cool

1) Turn off the power to the unit. There is normally a shut-off or disconnect panel on the wall next to the outdoor compressor. Also shut off the air conditioner’s 240-volt circuit at the main electrical panel.

2) Rake leaves and debris away from the outdoor condenser. Trim any bushes that might block airflow.

3) Unscrew and remove
protective grilles and the top cover or grille from the compressor. If the fan is attached to the grille, be careful not to pull any wires loose.

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4) Use a soft brush to clean dirt and debris from the fins, and then vacuum the fins with a brush attachment (taking care not to damage the fins).

5) From inside the unit, use a hose and nozzle with a trigger grip to spray debris from the fins (protect the wiring and motor with plastic sheeting or a large plastic garbage bag). If your owner’s manual calls for lubricating the motor, do that now—but don’t overlubricate.

6) Reassemble the unit.

7) To test it, turn the thermostat to Off, reset the power at the disconnect by the compressor and the main panel, and then set the thermostat to turn the unit on. To avoid straining an air conditioner’s compressor, wait at least five minutes between turning it off at the thermostat and turning it back on. Let it run for a few minutes, then feel the two pipes that connect the condenser unit to the air handler (slide any insulation back). One should feel warm, the other cool.

For more on this, see How to Prepare Your Air Conditioner for Summer.

If this does not work, call a qualified air-conditioning technician—the refrigerant may need to be recharged.

Air Conditioner Does Not Turn On

If your central air conditioner doesn’t go on automatically:

1) Be sure the thermostat is set to Cool and that the set temperature is well below the ambient temperature.

2) A central air conditioner should be on a dedicated 240-volt circuit; check the main electrical panel and any secondary circuit panels for a tripped breaker or blown fuse. If you find the problem there, reset the breaker or replace the fuse.

3) Make sure the furnace power switch is turned on and that the outdoor condenser’s power switch, which is mounted on the outdoor unit, hasn’t been shut off. Also be sure the 240-volt disconnect next to the compressor, which is in a metal box usually mounted on the house wall, hasn’t been shut off.

4) Turn off the power to the air conditioner and check the thermostat. Remove the thermostat’s cover and unscrew the wire from the Y terminal. Turn the power back on. Holding the wire by its insulation only, touch the bare end to the R terminal and hold it there for about two minutes. If the compressor kicks on, the thermostat is faulty; replace it or call an air-conditioning technician. If the compressor doesn’t go on when you hold the two wires together, turn the power back off and call a technician.

Room Temperatures Experience Major Swings

When room temperatures swing more than about 3 degrees F. between when the air conditioner goes off and on again, it generally means that the pump isn’t cycling on often enough. When room temperatures drop lower than the set temperature on the thermostat, it usually means that the thermostat is improperly calibrated or installed where it doesn’t sense a proper sampling of room air. See How to Adjust a Thermostat Heat Anticipator.

Air Handler Makes Noises

Though most air handlers have direct-drive motors, some older units may be belt-driven. Squealing sounds from a belt-drive air handler generally occur when the belt that connects the motor to the blower slips. If a direct-drive blower is squealing or making a grinding noise, shut off the unit and call an HVAC repair technician—the motor’s bearings are probably shot.

Water Pools Next to the Air Conditioner

Air conditioners and high-efficiency combustion furnaces create significant condensation, which exits through a plastic drain tube. This should go into a floor drain or be carried away by a small “condensate pump.” If water is pooling at the base of the appliance, something may be blocking the water’s flow or the pump may not be working.

1) Look to see if one of the tubes is leaking. If it is, replace it.

2) Test the condensate pump by pouring water into its pan. If the pump doesn’t start, either it isn’t receiving power or it is broken. Be sure that it’s plugged in and test the circuit. If it’s broken, either get it repaired or replace it.

3) If the pump runs but doesn’t empty the pan, the ball-like check valve just before the discharge tube is probably stuck. Unscrew the check valve, loosen the ball inside, and look for an obstruction. If it appears that a condensation drain tube is clogged with algae, remove it if possible (you may have to cut it and replace it later with a coupling). Run a wire through it to clear it. To kill the algae, pour a dilute solution of bleach (1 part bleach to 16 parts water) through the pipe.

4) Ice may be blocking the tube. If this is the case, be sure the filter isn’t dirty. If the filter appears to be fine, the air conditioner’s refrigerant supply is probably low. Call an air-conditioning technician.

Air Ducts Are Noisy

Many heating/cooling ducts are metal so they conduct noise quite readily from the air-handling unit to your rooms. To break the conduction of sound, you can have a heating contractor insert flexible insulation ductwork between the heating/cooling system and the ductwork runs.

If you hear a pinging or popping sound coming from the ductwork, this may be caused by thermal expansion or by air blowing past a loose flap of metal. Track along the duct runs, listening for the sound. If you find it, make a small dent in the sheet metal to provide a more rigid surface that’s less likely to move as it heats and cools.

Central Air Conditioner Repair

Central Air Conditioner Repair

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