Why Is My AC Freezing Up?

Here's the paradox: an air conditioner freezes up precisely when it can't absorb enough heat. The evaporator coil is supposed to run cold — just above freezing — while warm house air flows across it. When something disrupts that balance, the coil's temperature drops below 32°F, condensation on it turns to ice, and the ice insulates the coil further in a vicious cycle. The result: a block of ice where your cooling should be.

Almost every freeze-up traces to one of two root causes: restricted airflow or low refrigerant. Here's how to tell them apart and what to do.

Cause #1: Restricted airflow

Low airflow means less warm air reaching the coil, so the coil gets colder and colder. The usual suspects:

• Clogged air filter. The number-one cause, full stop. A filter that looks like a gray blanket is strangling your system.
• Closed or blocked vents. Closing vents in unused rooms doesn't save energy — it raises static pressure and reduces airflow across the coil. Keep supply vents open and unblocked by furniture.
• Dirty evaporator coil. Years of dust bypassing cheap filters cakes onto the coil fins, insulating them.
• Failing blower motor. If the indoor blower is weak or dead, almost no air crosses the coil. You may notice very weak airflow from vents before the freeze.
• Collapsed or disconnected ductwork. Common in Grand Strand attics — flex duct crushed by storage or chewed by pests.

Cause #2: Low refrigerant

Refrigerant isn't used up — low charge means a leak. With too little refrigerant, pressure in the evaporator drops, which drops its temperature below freezing even with normal airflow. Telltale signs: the system froze even with a clean filter and good airflow, cooling has been gradually weakening, or you hear hissing near the lines. This one needs a certified technician — federal rules require it, and the leak has to be found and fixed, not just topped off.

What to do right now

Found ice? Do this, in order:

1. Turn the cooling OFF at the thermostat — but set the fan to ON. Moving room-temperature air across the coil thaws it faster.
2. Check and replace the filter. If it's filthy, it may be your whole problem.
3. Open all supply vents and make sure the outdoor unit has clearance.
4. Wait. Thawing takes several hours — sometimes most of a day for a badly iced coil. Don't chip at the ice; you'll damage the fins.
5. Check the condensate drain while you wait — all that meltwater has to go somewhere, and a clogged drain plus a thawing coil equals water damage.

Once thawed with a fresh filter, you can try running it. If it cools normally, airflow was likely the issue — but keep an eye on it. If it freezes again, stop and call: the cause is deeper.

What NOT to do

Don't keep running a frozen system hoping it'll "push through" — ice blocks heat transfer completely, so you're paying for electricity to make ice, and liquid refrigerant can slug back to the compressor and destroy it. Don't pour hot water on the coil or attack ice with tools. And don't let anyone just add refrigerant without a leak search — you'll be paying for the same visit again next summer.

Why freeze-ups come back every summer

A one-time freeze after a forgotten filter change is a lesson learned. But some systems freeze every summer like clockwork — that's a chronic condition with a specific cause worth finding:

• A slow refrigerant leak. The classic repeater. The system gets topped off (or thaws and "recovers"), runs fine for weeks, then freezes again as charge drops. Only a leak search ends this cycle.
• Undersized ductwork or high static pressure. Some homes simply can't move enough air across the coil no matter how clean the filter is. The system was marginal from installation day.
• A failing metering device. The TXV or piston that meters refrigerant into the coil can stick or fail, flooding or starving the coil — both can ice it.
• Chronic low airflow from design. Too few return grilles is a common Grand Strand tract-home issue; the system is starved for air by architecture.

If you're on your third thaw-and-hope summer, stop hoping. A technician measuring static pressure, superheat, and subcooling can identify which of these is yours in a single visit — and the fix is usually cheaper than another summer of frozen weekends.