Why Does My AC Freeze Up?

The Physics in Plain Terms

Inside the indoor coil, refrigerant boils at a low temperature and absorbs heat from the air blowing across it. The system is designed so that the coil surface stays above freezing while doing this. Two things keep it there: enough warm air moving across the coil, and enough refrigerant at the right pressure inside it.

Remove either one and the coil surface drops below 32 degrees. Moisture condensing on it freezes instead of draining, ice accumulates, airflow drops further, and the freeze accelerates. That feedback loop is why a small restriction can end with a solid block of ice.

Airflow Causes, Ranked by How Often They Appear

• A clogged air filter — by far the most common
• A dirty evaporator coil restricting air passage
• Blocked returns or too many closed supply registers
• A blower running at the wrong speed or failing
• Crushed, kinked or disconnected flexible duct
• Undersized return ductwork for the equipment installed

Refrigerant Causes

The distinguishing clue is often the pattern. Airflow-driven freezing frequently follows a missed filter change or appears in a home with known duct problems. Refrigerant-driven freezing usually accompanies a gradual decline in cooling over weeks and may repeat after previous refrigerant top-offs.

• Low charge from a leak in the sealed system
• A restricted filter drier or metering device
• A failing expansion valve
• Moisture contamination in the refrigerant circuit

A Third Cause: Running When It Is Cool Outside

Operating a standard air conditioner when outdoor temperature drops into the low 60s or below can cause the coil to freeze, because system pressures fall outside the design range. This happens occasionally in spring and fall when a thermostat is left at a low setpoint overnight.

How to Thaw Safely

Set the thermostat mode to OFF and the fan to ON. Room-temperature air moving across the coil melts ice faster than leaving everything off. Depending on the ice volume, allow anywhere from one hour to most of a day.

Put towels or a shallow pan under the indoor unit. Melting ice can produce more water at once than the condensate pan and drain can handle, which is exactly how a frozen coil turns into a ceiling stain.

Never chip, scrape or pry at the ice. Coil tubing is thin-walled and easily punctured, and a puncture converts an airflow problem into a refrigerant leak.

After It Thaws

Replace the filter, confirm all returns and registers are open, and restart the system. If it cools normally and does not refreeze, an airflow restriction was the likely cause and you should keep an eye on filter intervals. If it freezes again, stop running it in cooling mode and arrange service — repeated freezing risks returning liquid refrigerant to the compressor, which contributes to the most expensive failure on the system.