What Causes an Air Conditioner to Freeze Up?

ice on exterior ac unit

What Causes an Air Conditioner to Freeze Up?

You would think that with an AC, freezing is good and will produce cold air. Unfortunately, the reality couldn’t be further from the truth. An AC works through a refined control of temperature, pressure, and airflow. If any of these things fall out of balance, the refrigerant system that creates the cold air can overclock and freeze itself, without actually making your air any colder.

A frozen AC will either blow warm/cool air or nothing at all. Of course, you’re wondering why this might be true and we’re here to explore the answer in-depth along with what you should do about it.


The key element to an AC is the coil full of extremely cold refrigerant. When activated, the refrigerant actually needs warm air to blow across them. This cools your air and keeps the coils warm enough that they do not freeze over. On a straight cool system, your indoor coil removes heat from the inside and transfers it using the refrigerant to the outdoor coil which pushes the heat to the outside. This is why your outdoor unit feels like it’s blowing noticeably hot air, hotter than the outside temperature. Because the system is pulling heat from your indoor air and transferring the extra heat outside.

It does this by manipulating pressure via the refrigerant circuit. But if something stops the warm air from blowing over the coils or if the pressure is wrong inside the coils, then the coils can become too cold and can quickly freeze up. The reasons why the inside of your AC can become a frozen-up icebox relate to either airflow, pressure, or both. Let’s break it down cause-by-cause.


When humidity is high in the air, this only makes the freezing problem worse, as ice starts to form on the coils and begins to insulate them with cold so that any available warm air can’t reach the coils to warm them. The more moisture there is in the air, the more ice builds up. It covers the inside of the AC like an old chest freezer and crawls up the copper pipes. We get the majority of our frozen AC calls during Monsoon season when homeowners begin to see ice crawling up the copper piping all the way to the outdoor compressor.


The single most common reason why your AC can freeze up is lack of airflow. This can be caused by a number of different malfunctions ranging from the blower motor to the air intake. If anything in the airflow system stops warm air from flowing over the coils while the compressor continues running, your AC will start to freeze up and this only gets worse with time.

Collapsed Duct

Your ducts are how air moves through the house so if something blocks the duct, naturally air stops flowing correctly and this can block airflow to the coils. Even if the rest of the system is running smoothly, a collapsed or blocked air duct in your home (or more than one duct) can cause your AC to lose airflow. The coils begin to freeze because there’s not enough warm air to keep them at the optimal temperature.

Bad Blower Motor

The blower motor, naturally, matters a great deal in this system. Whether it’s the spinning motor itself or the blower motor’s run capacitor, if the blower fan stops blowing then the air stops flowing quickly enough over the coils. And once that air stops blowing, the coils start freezing. You may hear irregular or rattling sounds from your AC if your blower motor is in the process of failing.

Low Voltage to the Fan

Sometimes, the culprit is home electricity. ACs require electricity to a variety of high-power components to run which is why air conditioning becomes a central part of your summer power bill. If your fan or blower motor are under-powered while the compressor keeps running, this causes the same airflow freezing problem we’ve been talking about.

Clogged Air Filter

Lastly, there’s your air intake filter. Your AC air filter is not something that should be casually neglected, though it is all too easy to forget. That filter sits between your dusty home vent and the AC itself, keeping the air (and your compressor coils) clean. But when it fills up with dust, like any filter, it becomes a clog instead of a pathway. If your air filter is clogged, it slows down airflow to a crawl and no amount of blower efficiency can keep your coils from freezing.


Running without an air filter, a low-quality filter, or not changing your filter has another risk. Dust. AC refrigerant coils are often damp from moist air condensing on the cold surface. When dusty air blows past damp coils, it builds up into a thick blanket. That dust acts as an insulator, trapping the cold inside the coils themselves creating an excessive amount of freezing cold. This combines with more moisture which builds up on the dust and can create the ice-box effect.


Finally, there’s the refrigerant. The level of refrigerant inside your coils matters a great deal because of how your AC manages pressure. Ever wonder why that big unit outside is called a compressor? It is compressing refrigerant after releasing the heat it collects. Warm air converts or “flashes” liquid refrigerant into a gas which heats up and drops pressure. The compressor then, well, compresses it back into a liquid and pumps it toward the blower fan inside the house again.

However, if there isn’t enough refrigerant in the system, the “flashing” occurs too early due to the lack of pressure. The flashing process creates enough cold in that moment to condense moisture onto the coils which instantly freezes it. The more the flashing occurs early, the more ice builds up until it fills the AC and creeps up the copper pipes. This is why it is so important to maintain the proper refrigerant levels in your AC system.


Finally, there’s what you can do if you notice your AC blowing warm air and building up ice near the outdoor unit. If you see ice building around or near any part of your AC, your first step is to turn the thermostat off. Stop asking for cold air that isn’t coming. This will turn off the compressor and allow the refrigerant to stop getting colder.

While the AC is off, leave the fan on. Adding airflow can help to start melting the ice and reducing the problem.

Next, call an AC repair service immediately. This is a dangerous situation and can permanently damage your system unless the cause is found and corrected. On the phone, let your service know that you’re calling because your AC is freezing up. This is an urgent call and will help them prioritize your service quickly.

Check your air filter and change it if it’s full. If this was the problem, or part of the problem, tell your technician when they arrive so they can better diagnose and solve your problem. Don’t worry. A frozen AC can be fixed, especially if you turn off the compressor and call for service quickly.

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