The short answer
A frozen AC almost always means 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 the vents. If it freezes again after a full thaw, there's likely a refrigerant leak or a blower problem. Don't keep running it frozen: liquid refrigerant can reach the compressor and damage it.
Why a coil freezes
The indoor coil is supposed to be cold, around 40 degrees, but not freezing. Warm house air moving across it keeps it above 32. When there isn't enough air, or the refrigerant pressure drops, the moisture in our humid air freezes onto the coil. Ice blocks more air, and it snowballs.
Blocked airflow
A clogged filter, a dirty coil, or a lot of closed vents can freeze a perfectly healthy system. Rule this out first.
Low refrigerant
Refrigerant isn't used up. If the charge is low, it leaked. Low charge drops the coil temperature below freezing.
A blower problem
If the outdoor unit runs but the indoor fan is slow or stopped, the coil freezes fast. A failing blower motor or capacitor is the usual cause.
How to thaw it safely
- Set the thermostat to off for cooling and the fan to on.
- Put towels or a pan near the indoor unit. A lot of water comes off at once and can overflow the drain pan.
- Replace the air filter while it thaws.
- Open supply vents and clear the return grilles.
- When the coil and lines are completely clear, turn cooling back on at a normal setting and check it again in a few hours.
When to call
- It freezes again after a full thaw and a new filter.
- The indoor fan isn't moving air while the outdoor unit runs.
- Melt water is overflowing the pan or showing up on a ceiling. Turn the power off at the breaker first.
Please don't add refrigerant from a store kit; it can overcharge the system and it doesn't fix the leak. Call Thermal Services at (704) 701-6478.
Common questions
How long does it take a frozen AC to thaw?
Light frost can clear in an hour or two with the fan running. A solid block of ice on the indoor coil can take several hours, sometimes most of a day. 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 copper line, and a hair dryer around wet electrical parts is dangerous. 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.
Is frost on my heat pump in winter the same problem?
No. Light frost on an outdoor heat pump in winter is normal, and the defrost cycle clears it every so often. Ice in summer, or a heat pump that stays encased in ice in winter, is a problem.