Homeowners call us baffled every summer: their AC is running, it feels warm inside, and there is ice on the refrigerant line coming out of the air handler or even on the condenser unit outside. In 116-degree Phoenix heat, ice feels impossible. But it is one of the most common service calls we take from May through September, and understanding why it happens will save you from making it worse.
Your evaporator coil works by absorbing heat from the air blowing across it. That absorption process requires enough airflow to keep the coil above freezing. When airflow drops — for any reason — the coil drops below 32°F and moisture in the air freezes onto the fins. Once ice builds up, it restricts airflow further, which causes more ice, which causes more restriction. Left running, a fully iced coil can damage the compressor because liquid refrigerant starts returning to a component designed to handle only vapor.
The two causes we find on almost every frozen-coil call
Cause one is a dirty or clogged air filter. This is by far the most common root cause in Phoenix. Desert dust and monsoon particulates load filters fast — a filter that was fine three weeks ago may be 80 percent blocked today. Pull yours right now and hold it up to the light. If it is gray and dense, replace it, let the system thaw completely with the thermostat set to fan-only, and restart cooling. Many frozen-coil calls resolve this way without a tech visit.
Cause two is low refrigerant charge from a slow leak. Refrigerant absorbs heat as it evaporates inside the coil. When charge is low, the pressure drops, the evaporation temperature drops below freezing, and the coil ices. This one requires an EPA-certified technician with gauges, a leak detector, and a refrigerant license. We find the leak — do not skip this step — repair it, and recharge to manufacturer spec. Running the system low on refrigerant without finding the leak means paying for refrigerant again in a season.
What to do right now if your coil is frozen
Turn the system off. Set the thermostat to fan-only to blow room-temperature air across the coil and speed the thaw. Do not chip or scrape the ice — you can bend the fins and puncture the coil. Most coils take two to four hours to thaw completely. Then replace the filter. If the system refreezes within a day of the filter change, call us — that is a refrigerant issue, not an airflow issue.
The real cost of letting a frozen coil run is compressor damage. Compressor replacement runs $1,500 to $3,000 and takes most of a day. A new filter costs $12. A refrigerant leak repair runs $225 to $490. The math is not complicated.