Why Is My AC Freezing Up?

A thin layer of ice covers the indoor coil or refrigerant line, a strange sight for an appliance meant to cool the house. In reality, freezing is a clear symptom that the system isn't working the way it should.

Why a system built for cold can freeze

An air conditioner works by circulating low-temperature refrigerant through a coil, over which the home's warm air passes to be cooled. This process only works well if enough warm air flows over the coil — without enough air, the coil's temperature drops too low and the moisture present freezes instead of simply condensing.

The most common causes

What to do the moment you notice frost

Shut the system off right away rather than letting it keep running — continuing to run a frozen system can damage the compressor, the most expensive part to replace. Let the system fully defrost, which can take several hours, before attempting anything else.

What you can check once it's thawed

Replace the filter if it's dirty — the most common and easiest cause to fix yourself. Check that every register is open and nothing is blocking return air vents in the home. If freezing recurs quickly after these checks, the problem is likely low refrigerant or a failing blower, both of which need a technician.

Why ignoring repeated freezing gets expensive

Running a system that repeatedly freezes speeds up compressor wear, a part whose replacement often costs nearly as much as an entirely new system. A prompt diagnosis at the first freezing episode avoids that risk.