Why Is My AC Not Cooling?

Understand what your system is actually doing

Cooling is heat movement, not cold production. Indoor air passes over a cold evaporator coil and gives up heat and moisture; refrigerant carries that heat outdoors, where the condenser releases it to the outside air. Both halves have to work, and enough air has to move across both coils.

That framing explains why so many different problems produce the same complaint. If the blower is starved by a dirty filter, the coil cannot absorb heat. If the outdoor fan is not running, the heat has nowhere to go. If refrigerant is low, less heat gets carried per cycle. In all three cases you feel the same thing: the system runs and the house stays warm.

Airflow problems

Airflow is the most common and most homeowner-addressable cause. A loaded filter, closed or blocked returns, furniture over registers, a dirty evaporator coil, or a weakening blower motor all reduce the volume of air passing over the coil.

The tell is often weak air at the vents even though the air that does come out feels cool. Left long enough, restricted airflow drops coil temperature below freezing and ice forms, which cuts airflow further until nothing moves at all.

• Replace the return filter and note how dirty it was
• Open all supply registers and clear returns
• Check for ice on the coil or the larger copper line
• Listen for a blower that sounds slower or louder than usual

Refrigerant problems

Refrigerant is not consumed during normal operation, so a low charge should be investigated rather than routinely topped off. A leak is the most common cause, though an incorrect charge left by previous service is also possible, and adding refrigerant without understanding why it is low may not solve the problem. Symptoms include gradual capacity loss across a summer, longer runtimes, ice at the coil or on the suction line, and higher-than-usual electric bills.

Charge has to be measured against manufacturer specifications using pressures, temperatures and superheat or subcooling calculations. It cannot be judged by feel, and topping off is not a repair.

Electrical and mechanical failures

If the indoor blower runs but the outdoor unit is silent or humming, the problem is usually in the condenser circuit: a failed run capacitor, a pitted contactor, a bad fan motor, or a compressor that will not start. The blower keeps circulating air, so the vents blow neutral air rather than nothing.

These components are behind panels carrying line voltage, and capacitors store a charge even after power is removed. Testing and replacement belong to a qualified technician.

Controls and thermostat

Before assuming equipment failure, confirm the thermostat is in Cool with a setpoint several degrees below room temperature and the fan on Auto rather than On. Fan set to On circulates air continuously, which feels like warm air whenever the compressor is off.

Dead batteries, a loose wire at the thermostat, a blown low-voltage fuse in the air handler, or a heat pump configured incorrectly all produce cooling complaints with nothing wrong in the equipment itself.

What a technician measures

A proper no-cool diagnosis is measurement-based rather than guesswork. Expect airflow and temperature split checks, static pressure, electrical readings on capacitors and motors, refrigerant pressures with superheat or subcooling, coil condition, and condensate system inspection.

Those readings distinguish between the causes above, which matters because the fixes range from a filter to a compressor.

• Return and supply temperature split
• Static pressure and blower performance
• Capacitor microfarads and motor amp draw
• Refrigerant pressures, superheat or subcooling
• Evaporator and condenser coil condition
• Condensate drain and float switch function