Why Does My AC Keep Freezing?

Why a coil drops below freezing

The evaporator coil is designed to stay just above freezing while a steady stream of warm indoor air passes across it. That airflow is what keeps the coil warm enough to shed condensation as liquid water instead of ice.

Reduce the airflow and the coil temperature falls. Reduce the refrigerant charge and the pressure in the coil falls, which also lowers its temperature. Either path ends in frost, then ice, then a fully blocked coil that stops cooling entirely.

Airflow causes

Airflow restriction is the more common cause and the one homeowners can influence. A loaded filter is the leading offender, followed by closed or blocked returns, a dirty evaporator coil, crushed or disconnected ductwork, and a blower motor that is no longer moving rated air.

Setting the thermostat extremely low does not help either — it produces long continuous runtimes that give ice more time to accumulate.

• Filter changed on schedule and correctly sized
• All return grilles open and unobstructed
• Supply registers open in most rooms
• Evaporator coil clean (technician inspection)
• Blower wheel clean and motor operating at rated speed

Refrigerant causes

Low charge lowers coil pressure and temperature, which freezes moisture on the coil surface. Because refrigerant is not consumed, a low charge should be investigated: a leak in the coil, line set, fittings or connections is the common cause, though charge lost or set incorrectly during earlier service is also possible.

A metering device problem — a restricted TXV or orifice — produces the same result by feeding the coil incorrectly. Both require gauges, superheat and subcooling measurement, and leak location work.

How to thaw the system safely

Turn the thermostat mode to Off, then set the fan to On so room-temperature air moves across the coil. Depending on how much ice has built up, complete thawing can take several hours or longer.

Never chip, scrape or pry at ice — the coil and its copper tubing are easily punctured, which turns an airflow problem into a refrigerant repair. Watch the drain pan while it thaws, since a large melt can exceed the drain's capacity.

1. Set the thermostat to Off
2. Set the fan to On to circulate air over the coil
3. Place towels near the indoor unit and monitor the pan
4. Replace the filter while the system is off
5. Wait until all visible ice is gone before restarting cooling
6. If ice returns, stop and have the system measured

Why it keeps coming back

Thawing addresses the symptom only. If the filter was the cause, a new filter fixes it. If the coil is dirty, the duct is restricted, the blower is weak, or the charge is low, the freeze recurs on the next hot day.

Repeated freeze-thaw cycles are also hard on equipment. Liquid refrigerant returning to the compressor and water overflowing a pan cause damage that costs far more than the original diagnosis would have.