Why Your AC Keeps Freezing Up

An air conditioner freezes up when the evaporator coil gets colder than freezing and moisture in the air turns to ice on it. That happens for two broad reasons: restricted airflow or low refrigerant. Less air moving across the coil means less heat picked up, so the coil temperature plummets. Low refrigerant drops the coil pressure and temperature for a different reason. Either way, ice builds up in layers, airflow drops further, and the cycle feeds on itself until the system can barely cool at all.

Airflow Problems Freeze Coils First

Before suspecting anything exotic, check the simple airflow killers. A dirty air filter is the number one cause we find on freeze-up calls in Brentwood. Next come blocked return grilles, closed-off supply vents, a blower wheel caked in dust, and ductwork that has come apart or collapsed in a hot attic. In larger two-story homes with long duct runs, even a few closed vents upstairs can starve the system of return air. We also see undersized ductwork in some older Brentwood homes, where the equipment was replaced with a bigger unit years ago but the ducts never caught up. Bigger is not better: an oversized system moves air it cannot push, and the coil pays the price.

Low Refrigerant: The Leak Story Again

If the filter is clean and every vent is open, the next suspect is refrigerant charge. Here is the myth correction that matters: refrigerant is never used up. A low charge means there is a leak somewhere in the sealed loop. Low refrigerant drops the pressure inside the evaporator coil, and lower pressure means lower temperature, which means ice. The tell-tale signs are hissing near the indoor coil or line set, ice forming even with a brand-new filter, and the system running longer and longer to do less and less. The fix is a leak search, a repair, and a weighed-in charge, not another top-off. On a typical visit we verify the charge with superheat and subcooling measurements, which tell us whether the system is low or something else is going on.

What to Do Right Now: Thaw It Out Safely

If you spot ice, do not keep running the cooling. Here is the safe sequence:

1. Turn the thermostat to OFF for cooling, then switch the fan to ON. Moving room-temperature air across the coil thaws it faster.
2. Replace the air filter while you wait, and open any vents you had closed.
3. Give it several hours, sometimes up to a day for a thick ice block. Never chip at the ice. Coil fins bend easily and refrigerant lines puncture easier than you would think.
4. Once thawed, try cooling again. If ice returns, stop and call. Continuing to run a frozen system can slug the compressor with liquid refrigerant, and compressors are built to pump vapor, not liquid.

Why Tennessee Humidity Makes Freeze-Ups Worse

Middle Tennessee air carries a lot of moisture, and every bit of it that hits a too-cold coil turns to ice. Homes on wooded lots, like many in Raintree Forest, deal with extra shade and moisture around the house, and their attic air handlers sit in the most humid part of the building. A system that would only sweat a little in dry country can build a full block of ice here. Clogged condensate drains add another twist: the drain pan overflows, the float switch kills the system, and the homeowner assumes the freeze-up is the drain's fault when the real culprit is still airflow or charge. We check both, because they often travel together.

Freeze-Up Cause Finder