Short answer: An AC usually freezes because too little warm air is flowing over the indoor coil, from a dirty filter, blocked vents or a failing blower, or because refrigerant is low from a leak. Turn the cooling off, run only the fan to thaw the ice, replace the filter, and call an HVAC technician if it freezes again.
Ice on the refrigerant lines, the outdoor unit or the indoor coil is a common summer problem. It stops the system cooling and can damage the compressor if it keeps running. This guide explains why your AC is freezing up and what to do.
What to do right away
- Switch the thermostat from cool to off.
- Set the fan to on so warm indoor air helps melt the ice. Thawing can take a few hours.
- Check and replace the air filter if it is dirty.
- Make sure supply vents and return grilles are open and not blocked by furniture.
- Place towels around the indoor unit, since melting ice can overflow the drain pan.
- Once all ice is gone, switch cooling back on and watch for ice returning.
Do not chip or scrape ice off the coil or lines. You can damage fins and refrigerant lines.
Common causes
- Dirty air filter: the most common cause. Less air across the coil lets it get too cold.
- Blocked vents or returns: closing too many vents or blocking returns reduces airflow.
- Dirty evaporator coil: dust on the coil blocks heat transfer.
- Low refrigerant: usually from a leak. Low pressure makes the coil colder than it should be.
- Failing blower motor or fan: weak airflow has the same effect as a dirty filter.
- Running when it is cool outside: many systems can freeze if run on cool nights.
Why low refrigerant needs a technician
If your system is low on refrigerant, there is a leak. Adding refrigerant without finding and fixing the leak is only a short term fix. Technicians who handle refrigerant must be certified under the EPA’s Section 608 rules, so this is not a do it yourself job.
When to call an HVAC technician
- The system freezes again after you replaced the filter
- You see ice on the outdoor unit or copper lines
- You hear hissing or bubbling, which can mean a leak
- The system runs but barely cools
- Water leaks from the indoor unit
Read about AC repair.
How to prevent your AC from freezing
- Change filters every one to three months, more often with pets or dust
- Keep vents and returns open and clear
- Have the system serviced every spring, including the coil
- Keep the outdoor unit clear of leaves and grass
- Avoid setting the thermostat very low on cool nights
Can a frozen AC damage the system?
Yes, if it keeps running. Liquid refrigerant can flow back to the compressor and damage it, and ice can bend coil fins. Turning the cooling off and thawing it as soon as you see ice protects the system.
Quick answers: why is my AC freezing up
Why is my AC freezing up?
Usually low airflow from a dirty filter, blocked vents or a weak blower, or low refrigerant from a leak.
How do I unfreeze my AC?
Turn cooling off, run the fan only for a few hours, replace the filter and check vents are open.
Can I add refrigerant myself?
No. Refrigerant work requires EPA certification, and the leak must be found and fixed.
How long does it take for an AC to thaw?
Often a few hours with the fan running, longer for heavy ice.
Who fixes a frozen AC?
An HVAC technician. Sifixit can connect you with a local HVAC company.
Related
This guide covered why is my AC freezing up. For more background, see ENERGY STAR’s heating and cooling advice, and if you need help, Sifixit can connect you with a local provider.
This guide is general information for homeowners, not professional, insurance or safety advice for your situation.
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