Short answer
AC coils freeze when too little warm air passes over them (dirty filter, closed vents, weak blower) or the refrigerant charge is low from a leak. Turn cooling off, run the fan to thaw, replace the filter, and call a tech if it freezes again.
Thaw it safely
Set the thermostat to Off and fan to On. Thawing takes 1–3 hours. Put towels near the air handler, since melting ice can overflow the pan.
Common causes
Airflow and charge account for almost every frozen coil.
- Dirty filter or coil
- Closed or blocked supply registers
- Failing blower motor or capacitor
- Low refrigerant from a leak
- Running the AC on cool nights below about 60°F outdoors
What the tech checks
Static pressure and blower operation for airflow, then superheat and subcooling for the refrigerant charge. If it's low, the leak should be found and repaired.
How to tell an airflow problem from a refrigerant problem
Airflow problems usually show up across the whole system: weak air at every register, a dirty filter, or a blower that sounds slower than usual. Refrigerant problems tend to return within days of thawing even with a fresh filter and open vents, and the system often loses capacity gradually over weeks.
- Freezes after a filter change: suspect refrigerant
- Weak airflow everywhere: suspect filter, blower or ducts
- Ice only on the outdoor suction line: often low charge
Prevent it from happening again
Most repeat freezes are avoidable with routine maintenance.
- Change 1-inch filters every 1–2 months in dusty climates
- Keep at least 80% of supply registers open
- Have the indoor coil inspected for dirt during each tune-up
- Don't run cooling when outdoor temperatures are below about 60°F
Why a top-off isn't a repair
Refrigerant circulates in a sealed loop and doesn't get used up. If the charge is low, there is a leak. Adding refrigerant without finding the leak only buys time and the coil will freeze again.
Questions
Is it safe to chip ice off the coil?+
No. The fins and tubes are thin and easily punctured. Let it melt.
How long does it take a frozen AC coil to thaw?+
Usually 1–3 hours with the system off and the fan running, longer if the ice is thick. Don't use a heat gun or hair dryer near the coil.
Can a frozen coil damage my compressor?+
It can. Liquid refrigerant returning to the compressor instead of vapor can damage its valves, so shut cooling off as soon as you see ice.
Why does my AC freeze at night?+
Cool nights reduce the heat load on the coil. Combined with a dirty filter or low charge, the coil temperature can drop below freezing. Raising the setpoint or switching off cooling on cool nights helps while you schedule service.
Editorial note: this guide is general information for homeowners, not a diagnosis. Desertline Heating & Air is a referral service; a licensed contractor should inspect your equipment before repairs.