When your AC unit freezes up in summer, it will quit cooling your home. The blower might keep running, but you probably won’t feel much cool air coming from the grates.
This guide discusses the causes of AC coil freeze up and how the issue can be solved. It starts with problems you can solve and moves to AC freeze up solutions that you’ll probably have to call an AC contractor to repair.
What Causes Air Conditioner to Freeze Up
When an AC unit freezes up, the cause will be one of these 9 issues. Here they are with brief explanations of why they cause the AC coil to freeze – and what can be done to solve it. OK, let’s answer the question – what causes air conditioner to freeze up?
The Common Causes of AC Freeze Up are:
- 1Dirty Air Handler or Furnace Filter
- 2Closed or blocked air vents
- 3Thermostat set too low
- 4Running the AC when it is cool outside (for dehumidifying)
- 5Dirty Evaporator Coil
- 6Low Refrigerant
- 7Bad Blower Fan
- 8Improperly Sized Ductwork
- 9Collapsed or Blocked Ductwork
Let’s explore each of these and the best solution for each.
1. Dirty Filter
Airflow – the lack of it – is the main cause of an AC coil freezing.
And a dirty filter is the primary cause of reduced airflow.
There’s more to this fix than filter replacement, so we encourage you to read it through.
Quick Overview on How AC works – We’ll stick to the main points.
The indoor coil is called the evaporator coil. It is connected to the outdoor coil, or condensing coil, by refrigerant lines. When refrigerant enters the evaporator coil, it evaporates into a gas and becomes less dense and cooler.
This is called the Joule-Thomson Effect.
However, as it evaporates, it has more capacity to collect heat. That’s the beauty of refrigerant. Think about how your skin gets cool as water evaporates off it, taking heat with it.
The refrigerant absorbs heat and carries it outside. The result is that the air around the coil is cooler. The blower blows the cooled air into your living space.
Removing heat makes the coil very cold. The only thing keeping the coil from freezing is the warm air being pulled in by the blower motor as it pushes out cold air.
When there is a lack of airflow:
- The coil gets freezing cold because warm air isn’t pulled over it in sufficient amount
- Moisture (humidity) in the air hits the coil, condenses onto it and freezes – and you have AC coil freeze up
Removing moisture from the air is a normal part of air conditioning. But when the coil is extra-cold and air isn’t flowing over it, the AC coil freeze occurs.
The Solution to central air conditioner freezing up, part 1: When you notice or suspect you have AC coil freeze up, turn off the AC by turning the thermostat to “Off.” If you leave it in AC mode, it won’t effectively cool your home, but ice will continue to build up on the coil.
Check the filter, and change it if it is dirty. If you don’t have a replacement filter, take the dirty filter outdoors and peel/pull some of the dirt/hair/debris off it like you might peel lint off a dryer filter. You can use a soft nylon brush for the purpose. Get a new filter and a backup the next time you’re out and about.
The Solution, part 2: Replace the filter, and turn the thermostat switch to Fan mode, and let the fan run. It will pull warm air over the and frozen evaporator coil, and the thaw process will happen more quickly.
Pro Tip: The ice might melt too quickly for the drain pan to handle the water – or a chunk of ice might block the drain hole. If the air handler or furnace is in an attic or on a wood floor, you might have to “babysit” the melt process. At a minimum, have towels handy to absorb leaking water before it can cause water damage. Periodically check the drain pan for an ice blockage, and remove it. You can do this by removing the access panel to the coil.
Pro Tip 2: When you see the AC coil iced up, you might be tempted to chip off the ice. We recommend allowing it to melt on its own. Chipping off the ice might damage the refrigerant line and/or bend a bunch of fins on the coil and make it less efficient. You’ve already got a headache. Minimize the pain by not creating a larger and more expensive problem.
If you change the filter but the AC keeps freezing up, there’s obviously another problem.
2. Too Many Vents Closed or Blocked
This is another cause of reduced airflow and can lead to your air conditioner freezing up inside.
Your home’s grates and vents are connected to two types of ducts – supply ducts and vents bring cooled/dehumidified air to the rooms in your home. The return ducts and vents pull in warm, humid air and carry it to the air handler to be cooled and dehumidified.
Supply vents usually have a lever on them, so they can be opened, closed or set partly open/closed. The reason is to allow you to control airflow into the various parts or rooms of your home. Sometimes, when it is really hot, a homeowner will shut grates leading to rooms that aren’t being used, so that maximum cool air can flow to rooms in use.
The problem is that if too many supply grates are closed, airflow is reduced or backed up in the ducts. The same problem can arise as with a dirty air filter. Not enough air is circulated over the coil. It gets too cold. Moisture condenses and freezes.
Return vents do not typically have levers for opening and closing them. They can’t be closed, but sometimes they get blocked by furniture, curtains or something like a box or bookshelf placed in front of them. Airflow is reduced, and conditions conducive to AC coil freeze up occur.
Solution: Turn the thermostat to the Off position or the Fan position, and allow the coil to thaw (see the Pro Tips above to prevent water damage from melting ice).
Open all your home’s air grates. And make sure return grates are unblocked. After the coil thaws and you turn on air conditioning again, you can close a grate or two. If there’s no problem and you want to close a few more, fine. But if the AC coil freeze up happens again, you’ll know the limits on how many you can close without a problem.
3. Thermostat Set Too Low
Some people like it really cold, often at night – and then they crawl under warm blankets! Are you one of them?
Or some central air conditioners, especially old ones or those with very dirty coils, don’t cool very well, so homeowners keep turning down the thermostat in hopes of getting relief from the heat.
Most air conditioners are capable of cooling to about 62F. If you set your thermostat lower than that, it might simply get too cold and cause the moisture condensing on it to freeze.
Solution: Obviously, turn off the AC. Setting the thermostat to Fan can help thaw the coil quicker. Be sure to see the tips above for preventing water damage from leaking water as the ice melts.
Pro Tip: If your central AC isn’t big enough, it might be forced to work too hard. AC coil icing might occur at a thermostat setting above 62, perhaps as high as 65 or 66. So, before you attribute the problem to something else, try keeping the setting above 66. If the AC keeps freezing up, then try another solution.
4. Running AC When it is Cool Outside
Who would run their AC when it’s cool outside?
Well, as you probably know, central air conditioning also dehumidifies the air.
Some climates are really damp in the spring/fall/winter, depending on region. Some homeowners want the dehumidifying effect and are willing to put up with the cooling. When the outside temperature is in the mid-60s or lower, there’s not enough warm air being pulled over the coil to prevent the coil from freezing.
Solution: Turn the thermostat to Fan, and allow the coil to thaw. Again, be sure to take precautions against water damage from melting ice (See above).
Then buy a dehumidifier or consider a portable AC or window AC with a Dry/Dehumidify Mode. These units pull heat and moisture from the air, but return the heat to the room while draining away the moisture. They’re ideal for cool, damp weather when you want the air in your home to be less humid and clammy. In short, they dry the air without cooling it.
5. Dirty Evaporator Coil
Why are the AC coils frozen? Could be as simple as they need to be cleaned. In this scenario, you might notice the AC pipe frozen too coming out of the coil.
This is a Top 3 reason an AC coil freezes up in summer. We place it here because some homeowners will want to call a heating and cooling company to resolve it.
The coil we’re discussing – the indoor coil – is the evaporator coil. We discussed why it is called this in the first AC freeze up cause.
The fins on the coil can become caked and clogged with dust, pet dander, fur or hair, moldy sludge or any other debris. When this happens, it insulates the coils, so heat cannot be effectively transferred through the fins and into the refrigerant. The refrigerant keeps making the coil colder, and it eventually reaches 32F or colder. Moisture condenses on it, and voila, the coil is soon disguised as a block of ice.
Solution: The evaporator coil needs to be cleaned. We’ve created a guide called Dirty Evaporator Coil? How to Clean Evaporator Coil. Cleaning an evaporator coil is a bit of a process, but our guide includes a step by step evaporator coil cleaning section showing how it is done.
You’ll have to allow the unit to thaw first, of course, but then you can tackle the cleaning job or hire a pro to do it.
6. Low Refrigerant
This is also one of the more common causes of AC freeze up. It doesn’t seem to make sense, but coil icing will occur with low refrigerant. Here’s why: As the refrigerant level drops, it is under less pressure. Less pressure means lower temperature. It will eventually drop below 32F. That’s the Joule-Thomson Effect again.
It won’t freeze, but it will cause the coil to get freezing cold, and the coil will freeze condensate as it removes moisture from the air.
Solution: Low refrigerant is evidence of a leak somewhere in the system. Refrigerant can leak through a pinhole, and it will evaporate. Or, it might leak out through a broken line or corroded fitting. In sort, you might or might not see evidence of a leak.
If you can’t determine the cause of AC coil icing, then this is a possibility. An AC professional can check the refrigerant level, locate a leak if there is one, and make a repair. If your AC coil is more than 15 years old, then coil replacement might be a better use of your money.
7. Bad Blower Fan
If the blower motor is burned out or the fan is somehow damaged, then the blower won’t produce proper airflow.
As we’ve seen from the first potential issue, airflow is one of the top reasons an AC coil freezes.
- Is the blower making noise – clunking or squealing?
- Is it not turning at all?
- Is a burnt-wire smell coming from the air handler or furnace?
These are clues, but you might need an AC technician to diagnose the issue.
Solution: It’s possible the blower motor or fan can be repaired. But often, bad blower motor replacement is needed.
Our Blower Motor Replacement Cost guide will prepare you for the bad news, if that is indeed the problem.
If the furnace is less than 10 years old, the part is likely under warranty. If so, the furnace company will provide the part at no cost, but you’ll still need to pay the labor cost of blower fan replacement.
Is that a surprise – that you might have to pay labor costs? We uncover the truth about HVAC warranties in this useful guide.
Of course, if you’re really handy and pay close attention to details and directions, you might be able to replace your own blower motor.
Pro Tip: If you DIY, be sure the motor you buy has the same specs as the one it is replacing. There are OEM parts and there are universal blower motors for some air handlers/furnaces. But it must have the power (1/6 horsepower to 1 horsepower) and volts. Look for one with the same number of speeds too.
8. Improperly Sized Ductwork
If an AC unit freezes up in a new home/new HVAC system or when the AC has just been replaced with one of a different size, this might be the problem.
And yes, it’s an airflow issue.
Ductwork that is too small for the AC and blower doesn’t allow enough air through the system to keep the coil from icing.
Ductwork that is too big doesn’t allow enough air pressure for sufficient airflow. AC freeze ups occur.
Solution / Pro Tips: If the home is new, then the builder should be liable/responsible for making the HVAC system right. Contact the builder first.
If the AC or coil is new, contact the people that put it in. Let them know that it is icing up. It might be the wrong size. Or, the technician might not have charged the new AC or coil with the correct level of refrigerant.
9. Blocked/Collapsed Duct
This is rare. It happens when ductwork is really old and is falling apart – literally. It could also occur if a racoon or other pest gets into the ductwork and builds a nest.
Again, it’s rare.
However, these problems will reduce airflow and cause a coil to freeze. Inspect visible ductwork. If you see damaged ductwork, call an AC company.
Solution: As with the other solutions, turn the thermostat to Fan and give the coil time to thaw. Guard against leaking water and the damage it can cause.
If you’re handy, you can try to repair it, but be careful. Sheet metal can be very sharp. If your ducts are coming apart, it’s likely time to have them replaced. Our Ductwork Design and Cost Guide is a good place to research this issue.
Still Freezing Up?
These 9 ways to fix a frozen AC unit are the most common reasons for AC freeze ups. But there are a few uncommon reasons. They’ll probably require diagnosis by a certified HVAC technician. If you’d like free quotes from top AC technicians in your area, there is no obligation for using our Free Local Quotes form or phone number. The AC companies are licensed, insured and pre-screened for experience.