Why Is My AC Running but Not Cooling?

How cooling works, and where it breaks down

An AC does not create cold air. It moves heat. The indoor evaporator coil absorbs heat and moisture from your home's air, refrigerant carries that heat outside, and the outdoor condenser coil releases it. If any link in that chain fails, the blower can keep running and pushing air through your vents while the air never actually gets cold.

Common causes of an AC that runs but blows warm air

Restricted airflow

A clogged filter, blocked return grille or dirty evaporator coil cuts the amount of air crossing the coil. With too little warm air passing over it, the coil can get cold enough to freeze, and then airflow drops even further. This is one of the most common reasons a system suddenly stops cooling in the middle of summer.

The outdoor unit is not doing its job

If the indoor blower is running but the outdoor fan or compressor is not, the air from your vents will be around room temperature or only slightly cool. Typical culprits are a weak or failed run or start capacitor, a worn contactor, a failed condenser fan motor, or a compressor that has tripped on a safety. A humming or clicking outdoor unit that never settles into a steady run is a classic sign.

Low refrigerant

Refrigerant is not used up like fuel. If the charge is low, there is a leak somewhere, often at a coil, a braze joint or a service valve. Topping off without finding and repairing the leak is only a temporary fix, and handling refrigerant legally requires EPA Section 608 certification, so it is not a homeowner task.

A dirty condenser or a heavy heat load

If the outdoor coil is matted with cottonwood fluff, grass clippings or dirt, the system cannot shed heat and cooling drops off, especially in the afternoon. Older, undersized or leaky-duct systems can also fall behind on the hottest days even when nothing is technically broken.

What you can safely check yourself

1. 1Confirm the thermostat is set to Cool, the fan is set to Auto, and the set temperature is several degrees below the current room temperature. If the display is dim or blank, replace the batteries.
2. 2Check the air filter. If it is gray, matted or you cannot see light through it, replace it.
3. 3Make sure supply vents are open and that furniture, rugs or curtains are not blocking return grilles.
4. 4Look at the outdoor unit from a safe distance. Is the fan spinning? Is the coil covered in debris? Clear loose leaves from around it and keep about two feet of open space on all sides. Never put your hands or tools near the fan.
5. 5Look for ice on the larger copper refrigerant line or on the indoor unit. If you see ice, turn the system off at the thermostat and set the fan to On to help it thaw. Do not keep running the AC with a frozen coil.
6. 6Check your electrical panel for a tripped breaker labeled for the AC or air handler. You may reset it one time. If it trips again, leave it off and call for service.

What a technician actually does to find the problem

A good diagnostic is a measured process, not a guess. A technician will typically confirm the thermostat call for cooling, check airflow and measure static pressure, compare the temperature of the air entering and leaving the evaporator coil, and test the capacitor, contactor and motors with a meter. They will also check the amp draw of the compressor and fans.

Only after airflow is confirmed do they read refrigerant pressures and calculate superheat and subcooling, which show whether the charge is correct. If the charge is low, a leak search follows, using an electronic detector, dye or a nitrogen pressure test. Repairing the leak, evacuating the system and weighing in the correct charge is the proper fix.

Houston and Gulf Coast factors

Our cooling season is long, and systems here often run for months with few breaks. That wears out capacitors, contactors and motors faster than in milder climates. Hot attics can push duct and air handler temperatures well above comfortable levels, humid air makes the coil work harder, and storm-season power surges can stress control boards and capacitors. A whole-home surge protector and a clean, free-flowing system go a long way toward preventing breakdowns on the hottest days.

If your AC is running but not cooling, shut it off rather than letting it labor. Continuing to run a system with a failing component or a frozen coil can turn a modest repair into a compressor failure.