The fan is running, air is moving through the vents, and the house keeps getting hotter anyway. In San Antonio in July, that is not a quirk, it is an emergency in progress. Here are the seven causes we find most often, in the order you should check them, starting with the ones you can fix yourself for free.
1. The thermostat is set wrong (check this first)
It sounds too simple, and it is also a real percentage of the service calls we run. Make sure the thermostat is set to Cool and not just Fan. On Fan, the blower circulates room-temperature air all day without ever cooling it. If the thermostat runs on batteries, replace them. If a family member and a smart-thermostat schedule have been fighting over the setpoint, that story ends here too.
2. A clogged air filter
A filter choked with dust strangles airflow across the indoor coil. The system runs, but barely any air passes through it, so barely any cooling reaches the rooms. Worse, restricted airflow can freeze the coil solid (see the next item). Pull the filter and hold it up to a light: if you cannot see light through it, replace it. During a San Antonio summer, check filters monthly.
3. A frozen evaporator coil
If you see frost or ice on the indoor unit or the copper lines outside, the evaporator coil has frozen. Ice on an air conditioner means something upstream is wrong, usually weak airflow or low refrigerant. What to do:
- Switch the system to Off and the fan to On to thaw the coil. Thawing can take several hours.
- Replace the filter while you wait.
- If it freezes again after thawing, stop and call a technician. Repeated freezing usually points to a refrigerant leak, and running the system that way risks the compressor.
4. A dirty outdoor condenser coil
The outdoor unit's job is to dump the heat pulled from your house. When its coil is caked with dust, grass clippings, and cottonwood fluff, the heat has nowhere to go and cooling collapses. With the power to the unit off, you can gently rinse the coil fins with a garden hose (not a pressure washer) and clear leaves and debris at least two feet around the cabinet. It is one of the highest-value pieces of maintenance a homeowner can do.
5. Low refrigerant from a leak
Refrigerant is not a consumable. A system never “uses up” its charge; if it is low, it leaked. Symptoms include warm air, ice on the lines, hissing sounds, and cooling that fades over days or weeks. This one is strictly a professional repair: the leak needs to be found and fixed, then the system recharged to specification. Topping off without fixing the leak is renting a repair, not buying one.
6. A failed capacitor or contactor
These small electrical parts start and run the compressor and fans, and Texas heat is brutal on them. A classic sign: the indoor fan blows fine, but the outdoor unit hums, clicks, or will not kick on at all, so the air moving through the vents never gets cold. Capacitors hold a dangerous charge even with power off, so this is a technician job, but the good news is it is one of the quicker and more affordable AC repairs.
7. Duct problems stealing your cold air
If the system cools some rooms but not others, or the air from the vents is cold but weak, the ducts deserve suspicion. Leaky or disconnected ducts in a 130-degree Texas attic can dump a large share of your cooling before it ever reaches a bedroom. A duct inspection tells you whether the system is failing or just losing the delivery battle.
When to stop troubleshooting and call
- Ice keeps returning after a thaw and a fresh filter
- The outdoor unit will not start, hums, or trips the breaker
- You suspect a refrigerant leak
- Anyone in the home is vulnerable to heat: infants, seniors, or medical conditions. Do not wait it out.
Polar Express repairs every AC brand across the San Antonio metro with upfront flat-rate pricing and a 24/7 emergency line. And if you want fewer of these afternoons, a spring tune-up catches most of these failures before July finds them for you: the San Antonio maintenance checklist walks through what that visit covers.
Frequently asked questions
Why is my AC running but the house is still hot?
The most common causes are a clogged air filter, a frozen evaporator coil, a dirty outdoor condenser coil, or low refrigerant from a leak. Start with the filter and the thermostat settings; if those check out, the problem usually needs a technician.
Should I turn my AC off if it is not cooling?
If you see ice on the unit or the refrigerant lines, yes: switch the system to off and the fan to on so the coil can thaw. Running a frozen or struggling system can destroy the compressor, which is the most expensive part in the machine.
How much does it cost to fix an AC that is not cooling?
It ranges from a free filter change you do yourself to several hundred dollars for common parts like capacitors or fan motors, up to four figures for refrigerant leaks or compressor problems. An honest diagnosis with an upfront quote should always come before any number.
