Why is my AC not cooling?

The unit is running, air is coming out of the vents, but the house just won't get cold. This is the single most common call we get in a Houston summer. Here's what's usually behind it, starting with the things you can check yourself in about five minutes.

1. A dirty air filter

This is the first thing to check, and it fixes the problem more often than people expect. A clogged filter chokes airflow across the evaporator coil. Less air means less heat gets removed, and in bad cases the coil drops below freezing and turns into a block of ice. In Houston, plan on a new filter every 1 to 3 months.

2. A frozen evaporator coil

If you pull the filter and see ice on the coil or the copper lines, the system needs to thaw before it will cool again. Turn the AC off but leave the fan on to speed it up. Ice is a symptom, not the disease. It's usually caused by restricted airflow or low refrigerant, so if it comes back you have an underlying problem.

3. Low refrigerant from a leak

Refrigerant isn't consumed like fuel. If you're low, there's a leak somewhere. Adding more without finding the leak just buys you a few months and vents refrigerant into the atmosphere. A proper repair means locating the leak, fixing it, then weighing in the correct charge and verifying it on gauges.

4. A dirty outdoor condenser coil

The outdoor unit's job is to dump heat outside. When the fins pack with grass clippings, cottonwood, and dust, it can't. Keep two feet of clear space around the unit and gently rinse the fins with a hose, never a pressure washer, which folds the fins over.

5. A failed capacitor or contactor

If you hear a hum from the outdoor unit but the fan isn't spinning, a failed start/run capacitor is a prime suspect. These are inexpensive parts and a common failure in our heat. This is a job for a technician, because capacitors store a dangerous charge even with the power off.

6. Thermostat or wiring problems

Confirm the thermostat is set to COOL and the fan is on AUTO, not ON. Fan set to ON blows room-temperature air between cooling cycles, which makes people think the AC stopped working. Dead batteries and loose control wiring cause their share of no-cool calls too.

7. Leaky or undersized ductwork

If your ducts run through a Houston attic that hits 140°F and they're leaking, you're paying to cool the attic. Weak airflow at the registers, rooms that never keep up, and surprisingly high bills all point this direction.

What you can safely check yourself

• Replace the air filter.
• Set the thermostat to COOL and the fan to AUTO; change the batteries.
• Check the breaker for the indoor and outdoor units.
• Clear leaves and growth away from the outdoor unit.
• Look for ice on the refrigerant lines. If you find it, shut the system off and let it thaw.