Short answer
An iced-up AC is almost always too little airflow or too little refrigerant. Turn cooling off and set the fan to on so the ice melts, then replace the filter and open vents. If it freezes again after a full thaw, there's likely a refrigerant leak or a blower problem, and it needs a technician.
What's probably happening
The indoor coil is supposed to be cold, but not freezing. It stays above freezing because warm house air keeps moving across it and because the refrigerant is at the right pressure. When either of those drops, moisture from the air freezes on the coil. Upstate humidity gives it plenty of water to work with. Once ice starts, it blocks more air, and it gets worse fast.
A dirty filter or blocked airflow
This is the first thing to rule out. A clogged filter, a dirty coil, or a lot of closed vents can freeze a perfectly healthy system.
Low refrigerant from a leak
Refrigerant isn't consumed. If the charge is low, it leaked out somewhere. Low refrigerant drops coil pressure and temperature, and the coil freezes. Topping it off without finding the leak only buys time.
A blower problem
If the indoor fan is running slow or not at all while the outdoor unit runs, the coil freezes quickly. A failing blower motor or its capacitor can do this.
Running it very cold on a cool night
A very low setpoint on a mild night can push a borderline system over the edge.
What you can check yourself in five minutes
- Set the thermostat to off for cooling, and set the fan to on. The moving air will melt the ice much faster.
- Put towels or a pan near the indoor unit. A lot of water will come off at once.
- Replace the air filter while it thaws.
- Open supply vents and clear the return grilles.
- Confirm the indoor fan is actually blowing. If the outdoor unit runs but no air moves inside, leave cooling off and call.
- Watch the drain pan and float switch. If the pan overflows or the float trips, the system will shut itself off. That's the safety doing its job.
- Once the coil and lines are completely clear, turn cooling back on at a normal setting and check it again in a few hours.
If it was a dirty filter, that may be the end of it. Put a reminder on your phone to check the filter every month through the summer. In a humid Upstate July, a filter that looked fine in May can be packed by August.
If you have a heat pump, a little frost on the outdoor unit in winter is a different thing. That is normal, and the defrost cycle clears it. Ice in summer, or a solid block of ice in winter that never clears, is not.
When to stop and call
- It freezes again after a full thaw and a clean filter. That points to a leak or a blower issue.
- The indoor fan isn't moving air.
- Melt water is overflowing the pan, coming through a ceiling, or reaching wiring or an outlet. Turn the power off at the breaker first.
- A breaker trips again after one reset, or you smell something burning. Turn it off at the thermostat and the breaker.
Please don't chip ice or add refrigerant from a store kit. If you're in Spartanburg or Greenville County, call Alpine Cool at (864) 702-0777.
Common questions
How long does it take a frozen AC to thaw?
It depends on how much ice there is. Light frost can clear within an hour or two with the fan running. A solid block on the indoor coil can take several hours. Let it thaw completely before turning cooling back on.
Can I chip the ice off or use a hair dryer?
No. Chipping can puncture the coil or the copper line, and a hair dryer near wet electrical parts is a bad idea. Turn cooling off, run the fan, and let it melt on its own.
Why does my AC freeze up at night?
Cooler nights and a low thermostat setting can push the coil below freezing, especially if airflow is already weak from a dirty filter or the system is low on refrigerant.
Will the melting ice cause a leak?
It can. A lot of water hits the drain pan at once as the coil thaws. Put towels down, and if the pan fills up or water shows up on the ceiling, turn the power off and call.