Switch the cooling off and let the ice melt on its own. Chipping at it can puncture the coil and turn an airflow problem into a refrigerant leak.
Why a coil freezes
The indoor coil runs cold so it can pull heat out of the air. As long as enough warm air crosses it, it stays above freezing. When the airflow drops or the refrigerant pressure falls, the coil gets colder than it should, and moisture from the air freezes on it instead of dripping away.
Likely causes
- A clogged air filter
- Closed or blocked supply vents and return grilles
- A dirty indoor coil
- A blower fan that is slipping, weak or failing
- A refrigerant leak that has left the system low
What to do right now
Set the thermostat to off so the compressor stops. Where the thermostat allows it, setting the fan alone to on moves room air across the coil and helps it thaw. Expect a lot of water as the ice melts, and keep people clear of any that spreads near the equipment or outlets.
Check the filter while the system thaws, and replace it if it is dirty.
When professional help makes sense
If the filter was clean, or the coil freezes again after it thaws, the cause needs finding at the equipment. A frozen coil hides its own cause, so a technician usually diagnoses it once the ice is gone and the system has run for a while.