My AC is on but my house won't cool down. What's wrong?

1. The air filter is clogged

This is the easiest thing to fix, so start here. A dirty filter blocks airflow, and the AC runs without cooling much. If you haven't changed it in the last 1 to 3 months, turn the AC off at the thermostat and find the filter, usually near the air handler or at a return vent. If it's gray and clogged, replace it, then turn the AC back on and give it 30 minutes.

2. The thermostat settings are off

Check this before you call, especially after a power outage. Make sure it's set to Cool, not Fan Only or Heat, and at least 5 degrees below the room temperature. On a smart thermostat, check that it hasn't been reprogrammed or put into an energy-saving mode.

3. The outdoor unit is dirty or blocked

The outdoor unit releases heat from your home, and it can't do that when it's surrounded by leaves, grass clippings, dirt or shrubs. Turn the AC off at the thermostat and clear at least 2 feet of space on all sides. If the metal fins are clogged, rinse them gently with a garden hose from the inside out, and let them dry before turning the system on.

Don't use a pressure washer, which bends the fins and damages the unit. Cottonwood season in late spring can clog outdoor units quickly around here, so check yours each year.

4. The system is low on refrigerant

Without enough refrigerant, the AC runs constantly but never cools the house. Signs include:

• Ice on the copper lines or the indoor coil
• A hissing or bubbling sound near the outdoor unit
• A house that never reaches the set temperature
• A higher electricity bill than normal

Refrigerant doesn't get used up, so low refrigerant means a leak. Recharging without fixing the leak is only temporary. This isn't a do-it-yourself repair: a technician needs to find the leak, repair it and recharge the system.

5. The evaporator coil is frozen

Ice on the indoor unit or its copper lines means the coil has frozen, usually from restricted airflow or low refrigerant. Turn the AC off and set the fan to On, not Auto, to help it thaw. This can take from 2 to 24 hours. Put towels around the indoor unit and replace the filter while you wait.

If it freezes again within a day or two, there's an underlying problem, such as low refrigerant, a failing blower motor or restricted ductwork, and it needs a diagnosis.

6. The AC is too small for the house

If your AC has always struggled on hot days, it may be undersized. An undersized unit runs nonstop, wears out faster and raises your electricity bill. This can happen when the original AC doesn't account for additions, finished basements or sunrooms. A load calculation by an HVAC technician determines the right size, based on square footage, insulation, windows, sun exposure and climate.

7. An electrical or mechanical problem

If you've checked everything above, the problem is likely inside the system, and it needs proper diagnostic equipment:

• A failed capacitor: the capacitor starts the compressor and fan motors. When it fails, the unit may hum but not cool.
• A bad contactor: the outdoor unit won't run, even though the indoor fan is blowing.
• Compressor failure: there's no cooling at all. Sometimes the cost points toward a new system instead.
• Ductwork leaks: cooled air escapes into walls and the attic, so some rooms stay warm.

AC repair