If the air smells of burning, or the outdoor unit hums without its fan turning, switch the system off at the thermostat and leave it off until a technician has looked at it.
How the system cools
A central air conditioner has two halves. Indoors, a fan blows room air across a cold coil, which takes heat out of it. Outdoors, a compressor and a second fan push that heat out of the house. Refrigerant carries the heat between the two in a sealed loop.
If either half stops doing its job, the indoor fan can keep running and blowing air that has not been cooled.
Likely causes
- The thermostat is set to heat, or the fan is set to on rather than auto, so it blows air between cooling cycles
- A clogged air filter is starving the indoor coil of airflow
- The outdoor unit is not running because of a tripped breaker, a failed capacitor or contactor, or a fan motor fault
- The indoor coil has frozen over and air can no longer pass through it
- The system is low on refrigerant because of a leak
- The outdoor coil is packed with dirt, leaves or cottonwood and cannot shed heat
What you can safely check
- The thermostat is set to cool, set below the room temperature, and has working batteries if it uses them
- The air filter is clean
- The outdoor unit is running when the thermostat calls for cooling, with its fan turning
- The breakers for the indoor and outdoor units have not tripped — if one trips again after a single reset, leave it off
When professional help makes sense
If the basics check out and the air is still warm, the cause is inside the equipment. A technician tests the outdoor unit’s electrical parts and measures the refrigerant pressures to find which part has failed. Adding refrigerant without finding the leak only puts off the same problem.