AC running but not cooling

When an AC runs but won't cool, the cause is usually one of five things: a dirty filter, a frozen indoor coil, a failed capacitor, a clogged outdoor coil or a refrigerant leak. You can check the first two yourself. The rest need a licensed tech.

First, is it actually broken?

On the hottest Tampa afternoons, a healthy system may not reach your setting. Systems here are usually sized to hold about 75°F inside when it's in the low 90s outside. If it's 96°F out and the house holds 78°F while the system runs nonstop, that may be normal. It should catch up after sunset.

A quick test: put one thermometer at a supply vent and another at the return grille, where air goes back in. A working system usually blows air 15 to 20 degrees cooler than the air going in. Much less than that and something is wrong.

Safe checks you can do

1. Check the filter. A clogged filter starves the coil of air. The coil gets too cold, ices over and stops cooling. Replace it and give the system an hour.
2. Look for ice. Check the larger copper line at the outdoor unit and, if you can reach it, the indoor coil. If you see frost or ice, set the thermostat to Off and the fan to On. Let it thaw for a few hours before you run cooling again. If it ices again, call.
3. Watch the outdoor unit. With the thermostat calling for cool, the top fan should spin and you should hear the compressor run. A loud hum with a still fan often means a dead capacitor. Don't push the blade to start it. The unit carries high voltage.
4. Clear around the outdoor unit. Cut plants back to give it about two feet on every side. Grass clippings and leaves on the coil stop it from shedding heat. A gentle hose rinse helps. A pressure washer bends the fins.
5. Check both breakers. The outdoor unit and the indoor air handler are usually on separate breakers. If only the indoor one is on, you get air but no cooling.

What a tech usually finds

A failed capacitor

The capacitor gives the compressor and fan motor the kick they need to start. In Tampa heat it's one of the most common summer failures. Signs: the outdoor unit hums but the fan won't spin, or it clicks and stops. A tech can test it in minutes. Expect about $150 to $400 installed.

A frozen indoor coil

Low airflow (a dirty filter, blocked return vents, a weak blower motor) or low refrigerant makes the coil ice over. The fix depends on which one. Thawing it only treats the symptom.

Low refrigerant from a leak

Refrigerant doesn't get used up. If it's low, it's leaking. Signs: ice on the lines, a hissing sound, warm air after a long run, and a system that worked after the last top-up and then faded again. Ask the tech where the leak is. A recharge without a leak search becomes a recurring bill.

A bad contactor or control board

The contactor is the switch that powers the outdoor unit. Pitted or stuck contacts stop it from starting, or keep it running when it shouldn't. Technicians say surges from Tampa's summer lightning storms also take out control boards.

A dirty outdoor coil

The outdoor coil dumps the heat pulled from your house. Coated in dirt, pollen or grass, it can't, and cooling drops on the hottest days first. A coil cleaning is routine maintenance and much cheaper than the parts it protects.

A failing compressor

The least likely cause and the most expensive. Signs include a breaker that trips each time the system starts, or a compressor that's hot but silent. On an older system, this is where the repair or replace math starts.

When to stop and call now

• A burning smell, smoke, or a breaker that trips again after one reset.
• Ice that comes back after thawing.
• Someone at home is elderly, an infant or medically fragile, and the house is above 85°F. A closed Florida house heats up fast.
• Water at the indoor unit as well as warm air. See AC leaking water.

Not-cooling questions