Why Does My Portable Air Conditioner Keep Freezing Up?

Written by: Paul Cathro

Updated on: December 26, 2022

Man studying air conditioner

If your portable air conditioner keeps freezing up, it's probably because of an airflow issue where cool air isn't whipping away from the coil fast enough.

Another common reason with new units is mis-sizing, where the portable AC is too large for the space – this can make it run short cycles, preventing moisture on the coils from evaporating and forming ice as a by-product.

There are several other reasons portable air conditioners can freeze up, which we'll run through below.

The good news is most issues are fixable with simple changes to how you install, set up, and use your portable AC.

Reasons Your Portable AC is Freezing Up (with Solutions)

These are the most common reasons for portable air conditioners freezing up:

7. Restricted Airflow

Evaporator coils can freeze solid without adequate airflow because air rushing past the coils whips cold air and moisture away from them.

Without this process, ice forms on the coil, and the coil temperature drops, eventually freezing up. 

The primary sources of restricted airflow include:

  • Blocked return air vent
  • Fan malfunction
  • Gaps in exhaust hoses
  • Poor unit placement (too close to walls)


Check the return air vent and all exterior vents for blockages (lint, dust, pet hair, etc.) and remove anything you find. 

Check the fan to ensure it is running correctly, and keep your portable AC at least two feet from the walls. If your portable air conditioner is too close to walls, it won’t suck in enough air to run correctly. 

6. Oversized Air Conditioner

On the face of it, an oversized air conditioner is an excellent idea because it can cool down your room quicker. However, oversizing a portable AC is not the best idea. 

While a more powerful portable AC will expel more cold air and cool down your room quicker, those shorter cycles of use (it will quickly turn itself off after reaching the target temperature) mean that moisture in the system has no time to evaporate and freezes onto the coils. 

Generally, you need 20 BTUs for every square foot of space – this means you need an 8,000 BTU portable air conditioner to cool 400 sq. ft.

Using an AC that is too big for a room will cause further problems too. It will run inefficiently as it will frequently turn on and off. It will also perform poorly when it comes to removing humidity.

If you are considering a new air conditioner, aim for 20 BTUs for every sq. ft. of space, so the unit is correctly sized. 

If you need to something smaller we have a great guide to the smallest portable ACs on the market.

5. Dirty Air Filters 

Probably the most common cause of frozen coils, dirty air filters reduce airflow into the system and hinder evaporation. 

Dirty filters obstruct air and create turbulence. Even after one month of use, air filters accumulate a surprising amount of dirt and should be cleaned periodically. 

We recommend monthly cleaning for air filters with mild soapy water and a soft brush (a toothbrush works well). If your air filters are replaceable, you should replace them at the manufacturer's recommended service intervals. 


  • Remove the air filter and clean or replace it.
  • If cleaning, blast it outside with compressed air and submerge it in a water-based detergent or soapy water for five minutes. 
  • Remove dirt with a fine brush – don’t use loads of pressure. 
  • Let it dry overnight. 
  • Install it into the portable air conditioner and run as usual. 

4. Dirty Coils 

Dirty evaporator coils are prone to freezing because dirt and grime on the coils promote the formation of ice crystals. This build-up of frost stops heat transfer, freezing the coils and rendering your portable AC useless. 

Your portable air conditioner is more likely to have dirty coils if the air filter is dirty, so it's a good idea to check both simultaneously. 


Inspect the evaporator coils to see if there’s an accumulation of dirt and grime. If so, you can clean them with a fine brush and a non-rinse detergent or degreaser. 

You can also buy a cleaning solution for coils, which the manufacturer might recommend for preventing corrosion.

3. Broken Evaporator Fan

While some fans break and stop working immediately, others chug away with reduced performance. It's easy to tell when a fan stops working because you can feel and hear it but diagnosing working faults is more challenging. 

Common faults include loose blades and housings, overworked fan motors, and defective wiring. Less common are faulty temperature sensors and defective relays, but you shouldn't discount anything when looking for problems. 

If your evaporator fan breaks or runs with reduced performance, the evaporator coils will freeze because there isn't enough air running through them to whip moisture away. 

If you still hear a fan, it could be the condenser fan if you have a unit with separate evaporator and condenser fans.


Listen to the fan to see if it makes strange noises or runs intermittently. Call the manufacturer if you notice anything strange. 

Fans are usually replaceable, but you might need to send your air conditioner to the manufacturer for repair. The most common fault is a loose screw in the housing, and you can check this by wobbling the mechanism. 

2. Refrigerant Leaks

A refrigerant leak in the coil is not uncommon in older air conditioners exposed to humid environments and incorrect chemicals. Leaks form due to small holes in the coils caused by erosion, which can form naturally or due to improper cleaning. 

If you have a refrigerant leak, your portable AC won't get to the temperature correctly, and the coil will freeze up in areas without refrigerant. 


Ask an air conditioning engineer to run a refrigerant pressure test to see if there is any loss of pressure when the system is charged. 

Unfortunately, coils are not usually serviceable, so you will probably need a new portable air conditioner.

1. Malfunctioning Thermostat

The thermostat is responsible for cycling your portable air conditioner after you set it by detecting changes in room temperature.

If the thermostat malfunctions, it can endlessly cycle, creating excess moisture on the coils that freezes. Additionally, if it runs short cycles, moisture in the system won't have time to evaporate, creating ice build-up.

While an uncommon problem, dodgy thermostats can freeze evaporator coils, so it's worth exploring the possibility. The simplest way to test the unit is to set the thermostat close to room temperature and see what it does.

Sometimes, people unfamiliar with your portable AC might also set the thermostat too high or low, affecting when it turns on.


Check the thermostat settings. If nothing is untoward, set the thermostat to one degree below the room temperature, so the unit boots up.

See if it cycles on and off appropriately in line with changes in room temperature. Something is up if it runs more often than it should or not enough. 

The manufacturer should be your first port of call for thermostat issues because they are not something you can fix unless incorrect settings cause them.

If your portable air conditioner is over five years old, it is also worth upgrading if you keep having problems with freezing coils.

A new portable AC will be more efficient, and you will get warranty repairs if anything goes wrong.

About the Author Paul Cathro

Paul is an ex-HVAC engineer with 5 years 'in the trade'.

He acquired tinyhousehugeideas.com in 2022 and aims to make it the internet's most comprehensive HVAC resource for small homes in the next few years.

You can learn more about Paul's story here.

Browse his published work on the website here.

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