Why Do Air Conditioners Freeze in Hot Weather?

June 4, 2021

An air conditioner freezes when pressure changes in the refrigerant coil change warm airflow or refrigerant amount in the coil, resulting in a freeze-up. Preventing this problem is as easy as scheduling routine air conditioner maintenance. An experienced technician will be able to detect the problem at the earliest opportunity.

It may seem counter-intuitive to think that your air conditioner can freeze over on a sweltering hot day, but that’s actually more likely than you think. In fact, hotter weather actually makes certain systems more prone to freezing over than in any other weather. Why does this happen and what can you do to avoid it and keep your home cool? Our blog will take a closer look at this topic, including explaining why air conditioners freeze and a few simple things you can do to keep your cooling system in peak condition so you can enjoy cooler air all summer long.

A Chilling Chain of Events

The simplest explanation as to why air conditioners freeze over during the summer months is simply that you use your air conditioner more when it’s hot outside. The hotter it is, the more your system will run, and thus the chances of your system freezing increase. However, the actual cause of your system freezing is more detailed than just the weather—it also has to do with the condition of your system.

Your air conditioner depends on a long length of metal tubing that contains a freezing-cold fluid known as refrigerant, known as a coil. Refrigerant’s job is to absorb as much heat as possible from the air that is forced over the metal tube. Any dirt or grime that has accumulated on the surface of your coil acts as a thin insulator that prevents this heat transfer from happening as efficiently as possible. If too much of this dirt and grime build up, your refrigerant can’t absorb enough heat to fully warm up to an expected level. As a result, it sits at a below-freezing temperature, and any water vapor that collects on the coil as a part of the cooling process then turns to frost. When enough of this frost builds up, the heat transfer stops completely, and eventually, your air conditioner needs to shut down entirely because it simply can’t operate any longer. At this point, you can typically open up your coil and find it completely covered with a thick layer of ice and frost that is difficult to remove.

What Causes Your Air Conditioner to Freeze?

What causes your air conditioner to freeze over like this? Obviously, this isn’t a normal problem that happens to every system—many are able to operate flawlessly during even some of the hottest and most extreme weather. There are two main factors that contribute to an air conditioner freezing over: airflow and a dirty coil.

First, as we explained before, dirty coils can’t absorb and transfer heat at the same rate as a clean coil, and thus the refrigerant that flows through the coil will constantly remain at a sub-freezing temperature. This causes any water vapor in the air that is pumped over the coil (typically a pretty high amount on a sweltering summer afternoon) to not only condense into water but quickly freeze into ice. The more vapor that flows over the coil, the quicker it condenses and freezes, creating a cascading, chain effect of your air conditioner slowly freezing over.

The second factor is the air flowing over the coil. In order to for your coil to function properly, it needs to absorb a certain amount of heat in order to warm the refrigerant to the point where ice and frost can’t form on the coil. It can’t do this if airflow over the coil is inhibited by an issue like a faulty blower fan or a wrongly sized air filter. However, arguably the biggest contributor to weakened airflow (and, simultaneously, to a dirty coil) is a dirty air filter. If you don’t change your air filter often enough, it will restrict the flow of air over your coil.

This is why we strongly recommend paying close attention to your air filter during the hottest months of summer. Make sure you change it right at the start of the hottest season of the year in order to start with a fresh filter that is free from dirt and dust to begin with. From there, check it every month. You may be surprised just how fast a filter can get dirty when running for hours and hours each and every day. Likewise, stock up on replacement filters at your local home improvement store and have a few sitting in storage for when you need them. You might be able to go all the way through summer on a single air filter, but you might need to replace it far more often than you think.

Finally, spend some time at the start of the season cleaning your air conditioner’s coil. Shut off the power to your cooling system, open up the door, and scrub the dirt and debris that has accumulated over the months off of your coil. This simple maintenance procedure can do wonders for increasing energy efficiency and reliability during the summer months.

Book your coil cleaning service or schedule an air conditioning repair by calling the pros at Winters® Home Services at 617-221-5899 today!