Why Is My AC Freezing Up?
by the Berry team · Updated
An AC freezes up when the indoor coil gets too cold, which almost always comes from too little airflow across it, like a clogged filter or closed vents, or from low refrigerant caused by a leak. Turn the cooling off and run the fan to thaw it, fix the airflow if that is the cause, and call a technician if it freezes again, because refrigerant problems are not a DIY job.
Likely causes and what they cost to fix
Ordered roughly from most to least common. Start with how to thaw it before diagnosing, since you cannot see much through the ice.
| Cause | How to recognize it | DIY cost | Pro cost |
|---|---|---|---|
| Clogged air filter | Weak airflow at the vents and a gray or dusty filter. The most common cause by far. | DIY$5-$40 for a filter | ProUsually included in a tune-up |
| Closed or blocked vents and returns | Several rooms closed off, or return grilles blocked by furniture. Too little warm air reaches the coil. | DIY$0 | ProNo repair needed once airflow is restored |
| Dirty indoor coil | Airflow stays weak with a clean filter. Common if filters were skipped for a long time. The coil may look matted with dust. | DIYNot recommended beyond a light surface cleaning | Pro$100-$400 to clean, more if the coil must be removed |
| Low refrigerant from a leak | Freezes again soon after thawing even with good airflow, and the air may be only slightly cool. Ice often starts on the larger copper line. | DIYNot a DIY job | Pro$200-$1,500+ to find and fix the leak and recharge, depending on the leak and refrigerant type |
| Failing blower motor or capacitor | Little or no air from the vents even though the outdoor unit runs. The blower may hum or run slowly. | DIYNot recommended | Pro$150-$400 for a blower capacitor, $400-$1,200 for a blower motor |
| Running in cool weather | Freezes on cool nights, especially below about 60 degrees outside. Most ACs are not designed to cool when it is that cool. | DIY$0 to raise the setpoint or open windows | ProNo repair needed |
Ranges are typical US prices and vary with your region, your home, and the season. A pro quote for your exact job beats any range.
How to thaw a frozen AC
- Turn the thermostat to "Off" for cooling, and set the fan to "On." The fan blows room air across the coil and thaws it faster than leaving everything off.
- Put towels down around the indoor unit. A thawing coil releases a lot of water, and the drain pan can overflow.
- Change the filter while it thaws, if it is dirty.
- Wait until the ice is fully gone. That can take a couple of hours, and sometimes most of a day. Running the cooling before it is thawed can damage the compressor.
- Turn the cooling back on and watch it. Check the copper lines near the indoor unit and the outdoor unit after an hour or two. If frost comes back, turn it off and call a technician.
Signs your AC is frozen
- Frost or ice on the larger copper refrigerant line at the outdoor unit or near the indoor unit.
- Air from the vents is weak or barely cool, even though the system is running.
- Water around the furnace or air handler, or a drain pan that suddenly fills, as the ice melts.
- A hissing or bubbling sound near the indoor coil, which can point to a refrigerant leak.
Why airflow matters so much
The indoor coil is supposed to be cold, but warm room air passing across it keeps it above freezing. When the airflow drops, from a clogged filter, closed vents, a dirty coil, or a weak blower, the coil gets colder and the moisture in the air freezes onto it. Ice blocks even more airflow, so it gets worse until the whole coil is a block of ice.
That is why closing a lot of vents to push air to other rooms can backfire. If you are trying to send more air to a hot second floor, see why the AC is not cooling upstairs for ways to do it without starving the system.
When it is the refrigerant
Low refrigerant also makes the coil too cold, and an AC is a sealed system, so low refrigerant means there is a leak. A recharge without fixing the leak is a temporary fix, and you will pay for it again. Ask the technician whether they found the leak, where it is, and what fixing it costs compared to the age of the system. Handling refrigerant requires EPA certification, so this is always a pro job.
If the system is older and uses R-22 refrigerant, recharging it is expensive because R-22 is no longer produced. A major leak on an old R-22 system is often the point where replacement makes more sense than repair.
When to call a pro
- The AC freezes up again after you thaw it and change the filter.
- The filter is clean and every vent is open, but airflow is still weak.
- You hear hissing near the coil, or a technician has recharged it before and it is low again.
- Water from the thaw has overflowed onto floors or ceilings, or reached electrical parts.
- The outdoor unit makes grinding, rattling, or loud humming sounds.
What makes it worse if you wait
- Running a frozen AC can send liquid refrigerant back to the compressor, which can damage the most expensive part of the system.
- Meltwater can overflow the drain pan and soak floors, ceilings, and insulation, especially if the air handler is in an attic.
- A refrigerant leak usually gets worse, and every recharge without a repair is money spent twice.
- Weak airflow from a dirty coil or failing blower makes the system run longer and costs more every month.
Common questions
How long does it take a frozen AC to thaw?
Usually a couple of hours with the cooling off and the fan on, and sometimes most of a day for heavy ice. Wait until all the ice is gone and the lines are dry before turning cooling back on.
Can a dirty filter cause my AC to freeze?
Yes. A clogged filter is the most common cause. It cuts airflow across the indoor coil, the coil gets too cold, and moisture in the air freezes onto it. A new filter often solves the problem for $5-$40.
Can I keep running my AC if it is frozen?
No. Turn the cooling off and run the fan until it thaws. Running a frozen system cools poorly, can overflow the drain pan as ice melts, and can damage the compressor.
How much does it cost to fix an AC that keeps freezing?
If it is the filter, $5-$40. A coil cleaning usually runs $100-$400. Finding and fixing a refrigerant leak and recharging the system often costs $200-$1,500 or more, depending on the leak and refrigerant. Blower motor replacements commonly run $400-$1,200.
Not sure that's your problem? Snap a photo and Berry will tell you what it is and what it should cost.
Keep reading
- How Often Should You Change Your Air Filter, and What Size Do You Need?How often to change a 1-inch or 4-inch HVAC filter, how to read filter sizes, which MERV rating to buy, and what a skipped filter change can cost you.
- Why Is My AC Not Cooling Upstairs?Why the second floor stays hot while downstairs is fine, the fixes to try first, and what airflow, duct, insulation, and zoning fixes cost DIY vs pro.
- How Much Does It Cost to Replace an AC Capacitor?What an AC capacitor replacement costs DIY vs pro, the signs a capacitor has failed, why the part is cheap but the job can be dangerous, and when to call.
- HVAC Maintenance: What to Do, How Often, and What It CostsThe full HVAC guide hub.