4 Causes of Your AC Freezing Up (And How To Fix It)

Hot summers have us running the A/C on full blast.

One of the side effects many people get from this is a frozen indoor coil.

You’d think all the ice would help keep things cool when it’s hot outside, but it actually does quite the opposite. Usually you’ll have warm air or almost no air coming out of your vents.

Don’t worry, this is an extremely common problem I see all summer long and as a licensed HVAC contractor I’ve got some info that can help you out!

Why Is My AC Freezing Up?

What causes A/C’s to freeze up can be something simple like a dirty air filter or a clogged evaporator coil.

Sometimes the cause can be something mechanical like leaking refrigerant or some part has failed. Unexpected low outdoor temps or even setting the thermostat too low can sometimes the coil to ice up.

There’s usually a quick fix for frozen ACs, but it can be a little more involved and you may need some professional help.

Keep reading to learn what causes the AC to freeze up…

Cause #1: Blocked Airflow

This is definitely the most common reason I find frozen units.

Plugged filters or coils prevent air from moving through your evaporator coil. Airflow is essential when running your A/C, because it exchanges heat from your house to the refrigerant that can moved outside.

Without the warm air from the house moving through the coil, the refrigerant temperature plummets.

The moisture on the coil from the humidity in the air begins to freeze and eventually creates a big block of ice. 

dirty furnace filter covered in fine dust

How To Fix?

The solution is actually pretty simple. Turn the A/C off and allow the unit to thaw.

It’s usually best to switch your fan to the “on” position to allow it to continue running and help thaw the unit faster. I wrote another article on how to thaw an A/C unit if you want more info on how to speed up the process.

After a few hours it should be thawed and you can replace the air filter. Make sure your filter is changed regularly and have your system tuned up and cleaned to prevent things from getting plugged up.

Cause #2: Mechanical Problems or Refrigerant Leaks

Air conditioners have a lot of moving parts and if not maintained parts can fail.

Fan’s can stop working, valves can stick or plug up, pipes can kink or leak, and any one of these things will cause a low pressure issue. Pressure and temperature are directly proportional, meaning they follow each other.

When the pressure goes down, so does the temperature. So this lack of pressure indicates a low temperature and when low enough, moisture freezes and creates a frozen A/C.

How To Fix?

This type of problem is typically fixed by a contractor or technician.

When dealing with refrigerants, there is a risk factor. Refrigerant is stored at high pressure and has extremely low boiling points at atmospheric pressure. That means it burns almost immediately if it touches your skin.

Here is our article on how to refill air conditioner refrigerant if you’re comfortable and confident in doing so, but I would recommend calling a professional to help diagnose and correct any refrigerant related issues.

Cause #3: Cool Summer Nights

Air conditioners have a range in which they can operate properly.

Low outdoor temperatures are another reason for your A/C pressures to drop and cause ice to form.

Most of the time if it’s below 60°F outside and your A/C is still running this can happen. 

air conditioning unit covered in layer of ice

How To Fix?

Shut your system down and allow it to thaw. If you see the temps falling outside, it’s best to open the windows and allow your house to cool naturally to prevent your A/C freezing up.

Some applications prevent this though, and you may need to run your system at lower outdoor temps. If that’s the case, look into installing a low ambient kit. 

This allows the outdoor fan to cycle and your compressor to build pressure and keep the A/C running when outdoor temps are cooler.

Cause #4: Thermostat Is Set Too Low

Some people like it cold inside and that’s okay! You just have to know your systems limits.

Try not to set your thermostat lower than 67°F.

Just like low outdoor temps, when indoor temp is too cold it drops the refrigerant pressure too low and ice begins to form.

How To Fix?

Allow the unit to thaw and keep your thermostat at a reasonable temperature. 67°F and above is generally okay and should not freeze.

Read Also >> What Is A Good Temperature For AC in Summer?

How Does An Air Conditioner Work?

Knowing how an air conditioner works can help you understand why the AC keeps freezing.

There’s a lot of technical information involved, so we’re just going to cover the basics and make it easier to understand.

Your air conditioner has two main parts, an indoor evaporator and an outdoor condenser, connected by copper lines that carry the refrigerant. The evaporator is absorbs heat from the air and moves it to the condenser where it is compressed and rejected through the coil.

In order for the refrigerant to absorb heat energy from the air, the temperature has to be lower than the temperature in the house, but above the freezing point of water.

That’s why when something goes wrong, it can easily start to freeze the moisture that collects on the coil and prevent your vents from blowing cold air.

Frequently Asked Questions

How Should I Stop My A/C Freezing?

Regular maintenance and filter changes are essential in preventing your A/C from freezing up. Make sure you have good airflow and allow your system to thaw completely if it does freeze before you turn it back on.

When in doubt call a professional and get things checked out.

Will My A/C System Unfreeze Itself?

It can fix itself if the temperature was set too low, or if it was running when the outdoor temps were too low.

Simply allowing it to thaw and turning it back on is all you need to do. If the cause was something else, then you may need to dig a little deeper and see what happened. Start with your air filter and follow the list above. 

What Is The Fastest Way To Thaw It?

Turn your thermostat to “off” to shut the A/C down and switch the fan to “on”.

Moving air through the coil helps the ice to thaw much quicker. If you really need it to thaw quickly, you can turn your furnace on and allow the heat from the furnace to help melt the ice.

Keep in mind this can cause a lot of water to leak out as it thaws and make a mess on your floor, so be prepared.

Read Also >> How To Check If Room Thermostat is Faulty?

Final Thoughts

Air conditioners freeze all the time so don’t fret.

Be patient when allowing it to thaw and make sure you’re doing preventative maintenance on your system. Airflow is essential in preventing your air conditioner from freezing up, so check your filter regularly.

If you’ve exhausted all solutions call a professional and get things looked at, but there’s a lot you can do to start with that may prevent you from having to call for help. A service call can end up just being a very expensive air filter!

References

Meet Your Home Inspector Secrets Author

join the 'save my home' newsletter

Get a weekly email every Tuesday with a single home maintenance tip to save, protect, and improve your house!