Why Do Air Conditioners Freeze?

Why Do Air Conditioners Freeze?


Frozen air conditioners are one of the most common problems we run across, even in the hot Birmingham summers. It seems completely counterintuitive…why would a machine freeze up when the temperature is over 100° F outside? The reasons are actually surprisingly simple, although if you didn’t read last week’s post on how air conditioners work, you may want to do so before going any further.

If you are all caught up on how an air conditioner operates, then you know that the basic concept of the system is that refrigerant travels through coils, and hot air blows across the coils to facilitate heat transfer (i.e., the warmer air becomes cool thanks to the refrigerant absorbing the heat). So, if something is off balance in the system, you can end up with a less-than-ideal heat transfer process. If the balance is thrown off enough, you can end up with heat transfer not happening fast enough and bam…next thing you know, the air conditioner is frozen. But what causes an unbalance in the system?


Dirty Air Filters

The most common cause of frozen air conditioners is a problem with airflow. Too little air blowing across the evaporator coils means that less heat is being transferred from the air to the refrigerant, which means the refrigerant can get too cold and cause everything to freeze. When filters aren’t changed on a regular basis, they can clog, making it very hard for air to flow freely as it should.

If your air conditioner has frozen up, the very first thing you should do is check your filter. Replace it if it is clogged and dirty, and remember to check your filter on at least a monthly basis to make sure it’s free of trouble-causing dust and debris.


Dirty Evaporation Coils

Another common cause of frozen air conditioners is dirty evaporation coils. This is another problem that can prevent air from flowing the way it should through your system – the dirt provides a barrier that prevents normal heat transference between the air and the refrigerant.

The evaporation coils are located inside your home, not in the outdoor a/c unit. If your filter looks good, this is the next thing you should check before calling an technician.


Ductwork Clogs & Dirt

If your filters and evaporation coils look fine, then the problem may be within the ductwork. Very dirty ductwork can sometimes cause a clog in the system, making it hard for air to flow freely and leading to a frozen air conditioner. To do a quick check, look at the vents and see if there is anything that could be causing a blockage, like dust or a piece of furniture in the way of flowing air. If you don’t see anything, it might be time to call a technician to come out and take a look at the problem. For very dirty ductwork, it’s probably worthwhile to have the whole system thoroughly cleaned.


Running A/C When It’s Chilly Outside

This is an easy fix – if the temperature is under 62° F outside, don’t run your air conditioner. The low temperature interferes with proper heat exchange between the refrigerant and the air, and the refrigerant never has a chance to warm up the way it should. If it stays cold, the pressure in your system drops, causing the refrigerant to just freeze up.


Too Little Refrigerant

This is a problem that definitely needs to be checked by an expert. If you have too much or too little refrigerant in your system, it can affect the pressure in your coils, and throw off the process of heat exchange and/or condensation of the refrigerant. In a nutshell, the coils become too cold due to too much expansion of the refrigerant, and the moisture in the air surrounding the coils begins to freeze.

If your system has too little refrigerant, it’s important to find out whether it just needs a refill, or if there is possibly a leak somewhere.


An air conditioner definitely should not freeze over and over again. If you have done a little DIY work, but the problem is still occurring, it’s time to call an expert.