If your AC evaporator coil is frozen, turn the system off at the thermostat, set the fan to ON, and let the ice melt completely before you do anything else. A frozen coil is almost always caused by one of two things: not enough warm air moving across the coil (a dirty filter, blocked vents or a weak blower) or not enough refrigerant in the system (a leak).
This guide explains why coils freeze, how to thaw yours safely, how to tell which cause you’re dealing with, and how to keep it from happening again.
What a frozen evaporator coil looks like
The evaporator coil sits inside your home, on top of the furnace or inside the air handler. You usually can’t see it without opening a panel, so most people notice the warning signs first:
- Little or no air coming out of the supply vents, or air that feels room temperature.
- Frost or ice on the larger copper refrigerant line running to the outdoor unit, often visible right where it enters the outdoor unit.
- Water pooling around the indoor unit or the drain pan overflowing as ice melts.
- The AC running constantly while the house gets warmer.
- A hissing or bubbling sound near the indoor unit (a possible sign of a refrigerant leak).
Why AC coils freeze
Your evaporator coil is cold on purpose: refrigerant inside it absorbs heat from the air your blower pushes across it. In normal operation the coil surface stays just above freezing, so moisture from the air condenses into water and drains away.
When the coil gets colder than 32°F, that moisture freezes instead. Ice builds up, blocks even more airflow, and the coil gets colder still. It’s a cycle that can turn the whole coil into a block of ice within hours.
Cause 1: Restricted airflow
This is the most common cause, and often the cheapest to fix.
- A dirty air filter. The number one culprit. A clogged filter starves the coil of warm air.
- Closed or blocked vents. Closing vents in unused rooms, or blocking returns with furniture, reduces total airflow.
- A dirty evaporator coil. Dust that gets past the filter mats onto the coil and acts like a second clogged filter.
- A weak or failing blower motor, or a blower wheel caked with dust.
- Collapsed or disconnected ductwork.
Cause 2: Low refrigerant
When refrigerant charge is low, pressure in the coil drops, and so does its temperature, below freezing. Low refrigerant always means a leak. Air conditioners don’t consume refrigerant.
Cause 3: Other causes
- Running the AC when it’s cool outside. Many systems aren’t designed to run when outdoor temperatures drop below about 60°F, and the coil can freeze.
- The thermostat fan set too low or a wrong blower speed setting after a repair or installation.
- A clogged condensate drain that lets water back up and freeze on the coil.
- A stuck contactor that keeps the compressor running even when the thermostat is satisfied.
How to thaw a frozen AC coil
- Turn the thermostat from COOL to OFF. This stops the compressor so the coil stops getting colder. Continuing to run a frozen system can damage the compressor.
- Set the fan to ON. Running the blower pushes warm indoor air over the coil and speeds up thawing.
- Prepare for water. Put towels around the indoor unit and check the drain pan. As the ice melts, a lot of water can come off the coil at once.
- Wait. Thawing usually takes 1 to 3 hours with the fan running, and up to 24 hours for a heavily iced coil. Don’t chip at the ice or use a heat gun. The coil fins and tubes are thin and easy to puncture.
- Replace the air filter while you wait.
- Open every supply vent and clear return grilles.
- Once all the ice is gone, switch back to COOL and watch the system for the next 24 hours.
How to tell which cause you have
After thawing, use these clues:
| What you notice | Likely cause | What to do |
|---|---|---|
| The filter was visibly dirty | Restricted airflow | Replace the filter and monitor. Often solves it. |
| Several vents were closed | Restricted airflow | Open all vents. Don’t close more than 10 to 20% of them. |
| Ice returns within a day with a clean filter | Low refrigerant or blower problem | Call a technician. |
| Hissing or bubbling sounds, rising bills | Refrigerant leak | Call a technician. |
| Froze during a cool night | Low outdoor temperature operation | Avoid running AC below about 60°F outside. |
| Weak airflow even with a clean filter | Blower, dirty coil or duct issue | Call a technician. |
When to call an HVAC technician
Call a licensed pro if:
- The coil freezes again after you’ve replaced the filter and opened the vents.
- You suspect a refrigerant leak. Only an EPA Section 608 certified technician can legally handle refrigerant, and the right repair is to find and fix the leak, not just add refrigerant.
- The blower seems weak or noisy.
- Water damage is occurring around the unit.
A technician will typically check refrigerant pressures, inspect the coil and blower, and test airflow. Leak repairs vary widely, from a few hundred dollars for an accessible leak to $1,500 or more if the coil itself needs replacing.
How to prevent your AC from freezing again
- Change your air filter every 1 to 3 months during cooling season. See how often to change your air filter.
- Keep at least 80 to 90% of your supply vents open.
- Keep return grilles clear of furniture and drapes.
- Schedule an annual tune-up that includes checking refrigerant charge and cleaning the coil.
- Keep the condensate drain clear. Our guide on how to unclog an AC condensate line shows how.
Frequently asked questions
Can I run my AC with a frozen coil?
No. Running a frozen system puts heavy strain on the compressor and can let liquid refrigerant reach it, which can cause expensive damage. Turn the AC off and thaw the coil first.
How long does it take for a frozen AC coil to thaw?
With the fan running and the compressor off, most coils thaw in 1 to 3 hours. A completely iced coil can take up to 24 hours.
Will a frozen AC fix itself?
The ice will melt if the system is off, but the underlying cause won’t go away by itself. If it was a dirty filter, replacing it usually fixes the problem. If it was low refrigerant, the coil will keep freezing until the leak is repaired.
Why is the copper line to my outdoor unit frozen?
Ice on the larger copper suction line is the same problem showing up outside: the coil and line are colder than they should be because of low airflow or low refrigerant.
For other reasons your AC might not be cooling, see our full guide: AC Not Cooling? 12 Causes and How to Fix Each One.
Disclaimer: this guide is for general information only. HVAC systems involve high voltage and refrigerant. When in doubt, contact a licensed HVAC professional.