AC Freezing Up: Why the Coil Ices and How to Thaw It

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

AC Freezing Up: Why the Coil Ices and How to Thaw It

Short answer

A coil ices for one of two reasons and they need opposite answers. Not enough air across it, which is the filter, the vents or the blower, or not enough refrigerant in it, which is a leak. Thaw it first with the cooling off and the fan on, then measure the temperature drop, because that one number tells the two apart for free.

An AC freezing up always comes down to the same thing, a coil running colder than it was meant to. What changes is why, and there are only two answers: not enough air going across it, or not enough refrigerant inside it.

Those two need opposite fixes, so guessing costs money. The good news is that one free measurement separates them.

Before you start

What to carry: two cheap thermometers, a new filter and some old towels. Ten minutes of work, then several hours of waiting for the ice.

Gas or electric? The cooling coil sits on top of a gas furnace or inside an electric air handler, and it behaves the same either way. What differs is only where you find the filter, so look in both the unit itself and any return grille in a wall or ceiling.

Switch the cooling off now. Before reading any further, put the thermostat on Off for cooling and the fan on On. Every minute it runs iced up is doing damage.

Why running it iced up is the expensive mistake. The coil is supposed to boil the refrigerant into a gas before it goes back to the compressor. A coil packed with ice cannot do that, so liquid refrigerant travels back down the line instead.

Compressors pump gas, not liquid. Liquid arriving is what breaks them, and a compressor is usually the point where a repair stops making sense against a new system. A cheap filter fault becomes a compressor replacement, and it happens over days rather than months.

Refrigerant is also not something to add yourself. It is not used up, so if it is low there is a leak, and topping it up without finding that leak is money into the air.

Thaw it first

Cooling off, fan on, towels down. Nothing else helps and several things make it worse.

Check 1Free, a few hours to a day

Cooling off, fan on, and leave it

Set the thermostat to Off for cooling but put the fan switch to On. That pushes room temperature air over the coil and melts the ice far faster than switching everything off.

Put towels under the indoor unit. A properly frozen coil holds a lot of water and the drain pan will not take all of it at once, especially if the drain is part of the problem: clearing a blocked drain line.

Bryant says anywhere from a few hours to a whole day, depending on how much ice there is. You cannot rush it, and the things people try to rush it with are the things that ruin coils.

No hairdryers held against the fins, no hot water, nothing sharp. The fins are thin aluminium and the tubing behind them is thinner.

Airflow, or refrigerant?

Two causes, two patterns. Ice over the whole coil is usually air, ice at one end creeping out along the pipe is usually charge.

Check 2Free, ten minutes

Measure the temperature drop once it is thawed

Run it for ten minutes, then put one thermometer in the return grille and another in a supply vent and take the difference.

Sixteen to twenty two degrees colder at the supply is normal. Over twenty two means the air is moving too slowly across the coil, which is an airflow fault. Under twelve means the coil is not absorbing what it should, which points at refrigerant.

That one reading decides which of the next two sections you are in. The same measurement is explained in more detail on the page for an AC that is not blowing cold air.

If it is airflow, and it usually is

Check 3Half an hour

Work the whole air path

The filter. Hold it up to a light. No light through it and you have found it. This is a common cause of a frozen coil.

The vents and the returns. Every vent open, including the rooms nobody uses, and the furniture and rugs off the return grille. A house with half its vents shut to push air upstairs freezes coils.

The coil itself. With the filter out, shine a torch at the fins. Grey felt across them means the dirt went past a filter that was left too long, and that is a cleaning job rather than a filter.

The blower. A wheel packed with dust moves a fraction of the air it should, and a motor running slow does the same. If the filter is clean and the vents are open and the drop is still over twenty two degrees, this is where it is.

Cool weather. Running the cooling when it is below about sixty outside ices coils on ordinary systems, because the pressures fall too low. If it froze on a cool night, that may be all it was.

If it is refrigerant

Low charge means a leak, because a sealed system does not use refrigerant up. Somewhere there is a hole, often at a joint made when it was installed, sometimes in the coil itself.

The pattern that goes with it is a system that has cooled a bit worse every summer, a drop under twelve degrees, and frost that starts at one end of the coil and creeps out along the big insulated copper pipe towards the outdoor unit.

This one ends with a technician and a leak detector, and the sensible question to ask them is where the leak is rather than how much gas it took. A system that is topped up every year is a system nobody has looked at properly.

On an older system it is also the moment to ask what refrigerant it uses, because the old gases are being phased out and the price of a recharge tells its own story about whether to keep going.

After it is thawed

Put a new filter in, open everything, and run it while you watch.

Take the temperature drop again after twenty minutes. If it is back in the sixteen to twenty two band, the airflow was the fault and you have fixed it.

If it ices again within a day, stop running the cooling and get somebody out. Two freezes in two days with a clean filter means charge or the blower, and neither improves by being used.

Questions people ask

How long does it take to thaw?

A few hours with the fan running, and up to a day if the whole coil went solid. Switching everything off instead of running the fan makes it slower.

Can I keep using it if I only turn it on in the evening?

No. Every run on an iced coil sends liquid back to the compressor, and that is the one fault that ends the system rather than costing a repair.

It froze once and has been fine since.

Then look at what was different. A filter changed just after, a cool night, or a door shut to a room. One freeze with an obvious cause is not the same as a pattern.

Ice on the outdoor unit in summer, not indoors.

Frost on the big copper line at the outdoor unit in summer is the same fault seen from the other end. In winter on a heat pump it is a different thing entirely: frost against a real block of ice.

Why does it freeze at night and not in the day?

Cooler outdoor air lowers the pressures, so a system that is marginal in the afternoon tips over after dark. It is a sign of a system that is already close to the edge.

Should I close vents in unused rooms to save money?

No. Trane says at least 75 percent of the vents in a house need to be open for proper airflow, and a Lawrence Berkeley National Laboratory study warns that closing too many can freeze the coil. The system needs a set amount of air moving, and closing vents takes it away.

Sources: Bryant "Why Does My AC Keep Freezing?" (dirty filter the most common reason, a few hours to a day to thaw, towels), Lennox "Why is My AC Unit Freezing Up?" (fan On to thaw, freezing below 60°F outside), Trane "Top 5 Causes of Frozen AC Coils" and "What To Do If the AC Evaporator Coil Is Frozen", American Standard "Got a Frozen Evaporator Coil? Don't Panic" (airflow the most common cause), Trane "8 Air Conditioning Myths" (75 percent of vents open), Lawrence Berkeley National Laboratory LBNL-54005 "Register Closing Effects on Forced Air Heating System Performance", Goodman service instructions RS6200006 (liquid floodback) and the DOE Building America "Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement" (temperature split)