Problem guide
AC unit freezing up
Ice on an air conditioner means the coil got too cold. Something cut the heat it was supposed to absorb.
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Written by HYRE HVAC Research Desk Primary-source research, data analysis and fact checking
The short answer
An AC unit freezing up means the indoor coil dropped below freezing. The usual causes are low airflow (a dirty filter, closed vents, a weak blower or a dirty coil) and low refrigerant from a leak.
Turn the cooling off, run the fan to thaw the ice, and replace the filter. If the ice returns after that, call an EPA Section 608 certified technician: the likely cause is refrigerant or a blower fault.
What to do now
Set the thermostat to off. Running a frozen system can send liquid refrigerant back to the compressor and damage it.
Set the fan to "on" so warm house air melts the ice. It can take a few hours.
Meltwater can overflow the pan. Put towels down and check the drain.
The most common cause.
Closed registers cut airflow.
Once fully thawed, turn cooling back on. Check the copper line at the indoor unit over the next day for frost.
Why the coil freezes
| Cause | How it causes ice | Who fixes it |
|---|---|---|
| Dirty filter | Less warm air across the coil | You |
| Closed or blocked vents | Same | You |
| Low refrigerant (leak) | Lower pressure makes the coil colder | EPA 608 certified technician |
| Dirty evaporator coil | Dust insulates the coil and cuts airflow | Technician |
| Weak or failing blower | Low airflow | Technician |
| Running on cool nights | Cool return air plus low load | Raise the setpoint or turn off |
Why low refrigerant causes ice
Refrigerant boils inside the evaporator coil at a temperature set by its pressure. When the charge is low, pressure drops and so does the coil temperature, below the freezing point of the moisture on it.
Ice builds, airflow drops further, and the coil gets colder still. DOE’s 2018 review of field studies found undercharge in a large share of installed systems.
A system that is low on refrigerant has a leak. Adding refrigerant without finding the leak only delays the next freeze. See refrigerant leak.
When to stop and call
Adding refrigerant, finding leaks or even attaching gauges requires EPA Section 608 certification.
Airflow or refrigerant fault.
Usually low refrigerant.
Do not open the outdoor unit, furnace cabinet or control box. Capacitors can hold a dangerous charge with the power off, and line-voltage work is for a licensed technician.
What a technician will check
Airflow first: filter, blower speed, coil cleanliness, duct restrictions and static pressure. Then refrigerant: pressures, superheat and subcooling against the manufacturer’s targets. If refrigerant is low, a leak search follows. ENERGY STAR’s checklist includes cleaning evaporator coils and checking refrigerant level as contractor tasks.
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Questions
Why is my AC freezing up?
How long does it take to thaw a frozen AC?
Can I pour hot water on the ice?
Is it bad to run a frozen AC?
Can a dirty filter really freeze the AC?
Written and audited by
HYRE HVAC Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a sales floor. We read the federal microdata file, the statute or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it.
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