Why is my AC freezing up?

Updated August 2026

Ice on an air conditioner almost always starts with airflow, and the leading cause is low airflow from a dirty filter or a grimy evaporator coil that drops the coil below 32F until moisture freezes on it. The first move costs nothing: turn cooling off, set the fan to ON, and let the block of ice thaw for a few hours before you do anything else. Then change the $15–40 filter and work down the causes below.

Safety first: a fully iced coil can take 3 to 24 hours to thaw and dumps that meltwater into the drain pan, so watch for an overflow while it clears. Do not restart the AC on ice: forcing the compressor to run against a frozen coil can slug it with liquid refrigerant and cause a repair north of $1,000. Never chip or scrape the ice off, which bends fins and can puncture the coil.

Causes, most likely first

What makes a furnace do this, most to least common (Updated August 2026)
Cause	How likely	The fix	Typical cost
Dirty filter or blocked returns	Most common	Low airflow drops the coil below freezing and moisture ices over. Replace the filter and open every supply and return vent.	Free (a $15–40 filter)
Dirty evaporator coil	Common	A grime-caked indoor coil insulates itself and frosts. A tech cleans it back to bare fins.	$150–400
Low refrigerant from a leak	Common	A low charge drops coil pressure and temperature until it freezes. Needs a leak repair and recharge, not a top-off.	$200–1,500
Failing blower motor	Occasional	A weak or dying blower moves too little air across the coil. The motor or its capacitor gets replaced.	$150–500
Running the AC below about 62F outside	Occasional	Cooling on a cold evening drops the coil past freezing. Wait for warmer weather or run the fan only.	Free

Check these first

Turn the thermostat to OFF and the fan to ON so the blower can thaw the coil. Forcing cooling onto ice only makes it thicker.
Replace the filter and open every supply and return vent in the house.
Once thawed, check the insulated copper line for oily residue, the fingerprint of a refrigerant leak.
Note the outdoor temperature. Running the AC below about 62F can freeze the coil all on its own.
Look at the indoor evaporator coil if you can reach it. A gray felt of dust on the fins insulates it and invites ice.

Prevent it next time

Freeze-ups are an airflow story, so the fix lives in the filter slot. Swap a 1-inch filter every 30–90 days and have the evaporator coil cleaned at the annual spring visit, part of the before-summer AC test. If the coil still ices with clean airflow, the charge is low, and a proper leak repair beats topping it up every single summer. A float switch on the drain pan is cheap backup for the meltwater a freeze produces.

When to call a pro

Once the coil has thawed and you have changed the filter, call a tech if it freezes again, if the copper line shows an oily stain, or if airflow feels weak with a clean filter. Those point at low refrigerant or a failing blower. A leak find-and-repair runs $200–1,500 and a blower motor $150–500. On a unit past 12–15 years that keeps icing from a leak, the replacement math may win.

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Frequently asked questions

How long does it take an AC to thaw?

A light frosting clears in an hour or two with the system off and the fan set to ON. A solid block of ice on the coil can take 3 to 24 hours to melt fully. Do not rush it with a hair dryer or a scraper; bent fins and a punctured coil cost far more than patience. Once every trace of ice is gone, change the filter, then restart and watch closely to see whether it ices up again.

Will a frozen AC damage the compressor?

It can, if you keep running it on ice. When the coil is frozen, refrigerant may not fully boil off before it returns to the compressor, and liquid slugging back is what wrecks compressors. That is why the rule is to shut cooling off and thaw before restarting. A slugged compressor is a $1,200–2,800 repair, against a free filter change, so a frozen coil is a stop-now signal, not a run-through-it one.

Why does my AC freeze up at night but not during the day?

Because the coil is closest to freezing when the load is lightest. On a cool evening, especially below about 62F outside, the coil temperature drops further with less heat to absorb, and any airflow or charge weakness tips it past 32F into ice. If it only freezes overnight, avoid running the AC when it is cool out, change the filter, and have the refrigerant charge checked if the pattern continues.