We run a lot of frozen-AC calls in July in the Pearland-Houston corridor. Nearly every one traces back to one of four things, and you can check three of them yourself in 10 minutes. Here is the order.
Restricted airflow over the indoor coil causes more of these than anything else, and a dirty air filter is behind most of that. It's the first thing our techs check on a frozen-coil call. Once airflow drops, coil temperature falls below 32°F and the moisture condensing on it freezes solid. The other three causes are a dirty evaporator coil and low refrigerant from a slow leak, plus a failing blower motor or capacitor. A leak search and recharge runs $350 to $650. Stop running the system the moment you see ice. An iced unit that keeps running can kill the compressor, and replacing one runs $1,800 to $3,500.
First: stop running it
If you can see ice on the suction line (the bigger of the two copper lines at the outdoor unit) or on the indoor coil, turn the system OFF at the thermostat and let it thaw for 2 to 4 hours. Running an iced-up AC burns electricity for nothing and can take out the compressor.
While it thaws, switch the thermostat to FAN ON. The blower pulls warm room air across the iced coil and melts it faster than leaving everything off.
The four real causes
1. Dirty air filter (the most common cause we see)
Your AC works by blowing room air across a cold coil. Clog the filter and airflow drops. With less warm air reaching it, the coil keeps getting colder until it's below freezing, and the moisture on it turns to ice. Ice then blocks what airflow is left. The rest of the coil freezes, and the line after it.
Check. Pull your filter out and hold it up to a light. If you can't see light through it, that's your problem.
Fix. Replace the filter. Let the unit thaw 2 to 4 hours, then restart it and watch for an hour. If it ices up again with a clean filter, move to step 2.
2. Dirty evaporator coil
A clean filter doesn't keep the indoor coil clean forever. Dust and biological gunk build up on it over the years, faster if the filter slot has gaps or the coil hasn't been serviced. The effect matches a dirty filter: less air across the coil, and it freezes.
Check. Shine a flashlight at the indoor coil through the filter slot. Gray or dark fins mean it's dirty. So do fins that look matted.
Fix. Coil cleaning is a real job: the access panel comes off, and the coil gets foam-cleaned and rinsed before it all goes back together. Either book a tune-up that includes coil cleaning, or a coil cleaning on its own. Don't try this with a hose; you'll bend fins and make it worse.
3. Low refrigerant (a leak)
This is the one homeowners can't fix. AC systems are sealed loops. They don't "use up" refrigerant. If you're low, you have a leak. As refrigerant level drops, pressure in the evaporator drops, and the boiling point of the refrigerant in the coil drops below freezing, even with perfect airflow. The coil ices up.
Check. Hard to verify without gauges. But if you've replaced the filter and the coil looks clean, and the system still freezes up, it's probably refrigerant. Ice that forms within a few minutes of startup points there even more.
Fix. We find the leak with an electronic detector and UV dye. A leaking service-port valve or fitting can be repaired, and so can a joint we can reach. When the leak is in the evaporator or condenser coil itself, the coil gets replaced. Adding refrigerant without finding the leak is a band-aid, and you end up paying for the recharge again next year.
4. Failing blower motor
If your blower motor has worn bearings or a failing capacitor, it spins slower than it was designed to. You get the same result as a dirty filter or coil: too little airflow, and the coil freezes.
Check. Stand near the air handler and listen with the system running. A healthy ECM blower is smooth and quiet. A failing one growls or whines. Open the access panel and watch the wheel. It should spin true without wobbling.
Fix. A capacitor replacement runs $180 to $320. A blower motor costs more, and the price depends on the model.
Less common causes (worth knowing)
- Kinked or crushed refrigerant line. Usually from rough handling during a recent install or a falling object. Looks low on refrigerant but isn't.
- Closed return vents. A homeowner closes too many supply registers (or a return vent gets covered by a couch), starving airflow. Surprisingly common.
- Thermostat set too low at night. Setting the thermostat below 68°F when outdoor temps drop into the high 60s pushes coil temps below freezing. Rare in Houston, but happens during cool fronts.
- Wrong-sized system (oversized). An oversized AC short-cycles, never reaches steady-state, and can ice up in mild weather. That's a design problem no service call will cure.
"If your AC is freezing up and you've changed the filter twice, please don't just keep running it and hope. We've replaced more compressors that died from running iced-up than from any other single cause. The repair on a frozen-up coil is much cheaper than the repair on the compressor that fails three days later."
The DIY decision tree
- Turn the system off. Switch fan to ON. Wait 2-4 hours.
- Replace the filter. Restart. Watch for an hour.
- Still freezing? Look at the coil. If dirty, schedule a cleaning.
- Coil looks clean and it still freezes? Call us. It's almost certainly refrigerant or the blower motor, neither of which you should DIY.
If it iced up again after a fresh filter and a full thaw, you're into refrigerant or blower territory. Same-day diagnostics: 281-992-7866 or book a tech.