Informational Guide

What Causes an Air Conditioner to Freeze Up?

There are several reasons why an AC unit may be frozen. Our guide showcases what causes air conditioners to freeze up and how to easily fix it.

by Romeo Abaquita

There is no worse fear on a hot summer day than turning on your air conditioner only to find out it isn’t working. To make matters worse, you find that ice has built up on your HVAC system when you go to investigate.

Frozen air conditioners can be worrisome, and the repairs can be costly, especially if you can’t diagnose why the system is freezing. There are several reasons your AC can freeze up, and this article will cover them all. We will help you find out why your AC is freezing and what you can do about it.

Before you can diagnose what is causing AC freeze up, you should have a basic understanding of how the system works. Knowing how everything goes together will help you diagnose where the problem is and which components or parts are causing it.

A central air conditioner has two main components, the evaporator and condenser. The refrigerant lines run between the two, which is where the heat is transferred from the air inside to the air outside.

The condensing unit takes the hot refrigerant gas and moves air over the coils, which remove the heat from the gas. The cooler gas is then sent through the compressor. Here it is compressed rapidly into a high-pressure liquid, causing it to heat up.

Once the hot liquid refrigerant leaves the compressor, it is sent through an expansion valve and filter drier. This releases most of the heat and begins the expansion process, where the liquid turns into a gas.

This semi-gas then enters the evaporator, where it forms into a cold gas that removes heat from the inside air as it travels through the evaporator coils. This causes the air to become quite cold and is sent by the blower through the ducting and out into the rooms of your home.

The gas in the coils heats up and is returned to the condenser, where the cycle repeats. What is most crucial to understand, though, is that the evaporator and condensing units are calibrated specifically for your installation, and the refrigerant is calculated by temperature and weight.

The evaporator will not heat more refrigerant gas than the condenser can handle, and the compressor will not send more liquid refrigerant than the evaporator can handle. If this process is ever disrupted, freezing can occur.

Signs That Your Air Conditioner Is Freezing Up

There are many signs and causes that your AC may be freezing up. Here we will cover these reasons and signs that your system is in danger of freezing up.

  • Actually freezing up.
    It is possible to see the evidence before you notice anything. Ice building up on the low-pressure line leading to the condensing unit is the first sign. It will then move through the condenser and compressor.
  • Lowered airflow.
    When the system is running, you may notice no air (or low volume) coming from the vents. This can happen suddenly, which is a clear sign of freezing.
  • Warm air from vents.
    If the evaporator is frozen, there can still be solid airflow, but the air will be warm. If your home is not cooling (or even getting hotter), freezing is a potential suspect.
  • System running constantly.
    If your AC never seems to shut off or turns back on right away, running in a near-constant state, it may be freezing up.

Reasons Why Air Conditioners Freeze & How to Fix Them

Now that you know the signs to look for, let’s take a look at the reasons those signs of freezing appear. There are quite a few, so proper diagnosis may not happen right away.

Low Refrigerant Levels

The only explanation for low refrigerant is a leak. As a sealed system, there is no other way for refrigerant to leave the refrigerant lines. However, a bad valve, pinhole in the refrigerant lines, or a faulty component can cause the refrigerant to leak out slowly.

When it gets too low, the refrigerant cannot compress enough or move through the system fast enough to maintain the heat exchange. This causes the low line and the coils to become colder and colder until ice forms. As the ice continues to spread, the system gets slower until it shuts down to freeze solid.

To fix this issue and prevent ice from forming again, you will need to locate the leak and repair it. This may take a trained HVAC professional using leak detection equipment and repair methods.

Drainage Problems

Another aspect of freezing systems is the drainage lines from the drain pan. While this isn’t the most common reason for freezing, it can be a factor. If the drain pan becomes clogged, the cold water dripping from the evaporator coils will collect in the drain pan and cause the heat exchange in the evaporator to slow.

If it goes on long enough, the cold evaporator coils will start to freeze and, over time, can encase the entire evaporator system. This is also one of the hardest to diagnose since the system will still run correctly for a while, and the ice can melt when the system is off, removing any trace of a problem.

To fix this issue, you need to unclog the drain, clean the drain pan and ensure proper drainage through the system. The use of drain pan tablets can help prevent future clogs.

Faulty Fan

Besides low refrigerant, the other major cause for frozen lines is low airflow. The problem with this diagnosis is that there are many reasons for low airflow in a system. One of those reasons, though, is a faulty blower fan.

If the fan in the evaporator unit is not running correctly, is intermittent, or has stopped altogether, then there isn’t any warm air being pulled over the evaporator coils. This causes the heat exchange in the evaporator to slow or even stop, and the cold refrigerant in the coils will cause ice to form. This will even further diminish the airflow, compounding the problem.

If the fan is faulty, replacing the fan motor is your only option, unless there is a secondary cause of the fault, such as loose or bad wiring or a bad run capacitor. It may take the inspection from an HVAC technician to properly diagnose.

Faulty Thermostats

Emerson Sensi Wi-Fi Smart ThermostatAnother uncommon but easily missed cause is a faulty thermostat. The thermostat can short out in the run position, forcing the system to keep running, even when the condensing unit has shut off. If the evaporator unit is still trying to cool the air and the compressor is not running, ice can form in the evaporator.

There are many reasons a thermostat can short or fault, but dust and low batteries (if equipped) are the most common. Replacement of a thermostat is a fairly simple DIY project, and you can also take the opportunity to upgrade to a smart thermostat.

Low Outdoor Temperature

Running the air conditioner when the ambient outdoor temperature is too low will also affect the heat exchange. If the refrigerant is cooled too much at the compressor, it won’t pressurize enough. A lower pressure refrigerant running through the high pressure line can also cause freezing.

When this happens, you will generally see the high line become frosty long before the low line starts to ice over. However, once the freezing reaches the evaporator, it only takes a few run cycles to reach the low line and compressor itself.

The only solution for this issue is to not run the air conditioner when the outside temperature is too low.

Dust Blocking The Coils

Dust and debris build up on the evaporator and condenser coils, and they must be cleaned regularly. However, even between cleanings, there can be enough accumulation to block proper airflow.

If the coils in either place become blocked enough and the airflow is impeded, ice will begin to form and spread throughout the system. Using coil cleaners, fin combs, and removing debris from the evaporator or condenser coils will usually be enough to fix this freezing issue.

Lack of Airflow

Just as blocked coils can impede airflow and disrupt circulation, so can a dirty or clogged air filter. This is the most straightforward problem to solve and diagnose. Simply remove the air filter and turn the system back on (only for a few minutes). If air flows properly and blows cold, replace the filter with a new one.

Other airflow issues can be caused by a number of other issues you may not see. Damaged or collapsed ducting, for example, or a faulty run capacitor not providing enough voltage to run the fan at a high enough speed. If you have clean coils and have replaced the air filter, and are still experiencing low airflow, you should call an HVAC tech in to diagnose the issue.

How to Fix Frozen AC Components

When you notice ice on the lines, pipes or components, the first thing you need to do is shut the system off. You will need to wait until the ice is completely melted and removed before attempting any repairs or replacements.

You also need to follow all safety and hazard warnings, protect yourself from electric shocks, and never work on the system with breakers, thermostats, and the system still powered on.

Air Conditioner Line Frozen

The air conditioner pipes, or copper lines, are usually (though not always) the first to freeze over. The low pressure line is already colder to begin with, and any drop in the flow rate of the refrigerant, pressure on the line, or drastic temperature change can cause them to freeze. You will notice this happening on the larger line leading to the condenser unit outside.

When this happens, the first thing you need to do is shut the system off. If the line is only frosted, you should check the easiest components first. Replace the air filter and inspect the thermostat for damage or shorting.

If the airflow is sufficient, it is most likely a leak in the refrigerant pipes, and you will need to locate the leak and repair it. An HVAC technician should be called to locate and perform the repairs on the leak. Once the leak is fixed, the technician can also refill the refrigerant to the proper amounts to prevent future freezing.

Air Conditioner Coils Frozen Over

When the condenser coils (outside unit) are frozen over, it can be because of a refrigerant leak, but the most common culprit is a faulty fan motor. You can check this quickly by leaving the system running and checking if the fan is spinning (visual check only).

If the condenser fan is not moving, the coils can freeze. This will also cause the compressor to overheat, which may trigger the overheat protection switch, shutting the compressor off. With the compressor off, the refrigerant doesn’t cycle, and ice begins to form.

A bad fan motor will need to be replaced. However, you need to ensure that the motor is the same size, rotation, wattage and output as the original motor. Replacing with a smaller motor or larger motor can result in overheating or icing just as easily.

Air Conditioner Frozen Inside Coil

If you notice a lack of airflow from the vents in your home, it is most likely a frozen coil in the evaporator. Opening the evaporator access panel will allow you to see the evaporator coils, and you will be able to tell right away if they are frozen or just dirty.

If they are frozen, you will need to defrost them so you can find the underlying problem. Leaving the system on in fan-only mode will help defrost faster while being less likely to overflow the drain pan.

Once defrosted, you can run the system with the access panel off to see if the blower fan is moving. If it is, the problem is most likely a leak in the refrigerant lines, which will need to be inspected and located by an HVAC technician. If it is not moving, you will need to replace the blower motor fan or hire a technician to do it for you.

Air Conditioner Indoor Unit Frozen

If the evaporator itself is frozen and not just the coils, the most likely cause is a faulty blower motor. However, blocked airflow is also highly probably and easier to diagnose. The first step is to remove the ice. Once the system is defrosted, you can better diagnose the issue.

Turning the AC back on after the ice has melted will tell you pretty quickly if it is an airflow issue or a leak. When running, check the air pressure coming from the vents. If there isn’t any, you can open the evaporator access panel to see if the blower fan is moving.

If it is, the problem is most likely a clogged filter or a collapsed duct. The filter you can replace yourself; however, it is better to have a technician diagnose and repair collapsed ducting or a leaking refrigerant line.

RV Air Conditioner Freezing Up

coleman rv air conditionerIf you experience ice in your RV’s air conditioner, the same reasons can apply. However, because the system is more compact, closer, and exposed to the elements, it is most likely a dirty external unit.

The first thing to check, though, are the easy things. Ensure that the filter is new, or at least not clogged or dirty. Double-check that the exhaust vents are also cleaned as debris and dust can block airflow exiting the system.

Finally, before climbing on the roof, check that the thermostat is in working order and everything comes on when the thermostat is used. If all of that fails, you should remove the shell of the top mount unit and inspect the coils for dirt and debris build-up blocking airflow. Clean the system according to the manufacturer’s specifications and try the system again.

If it still continues to freeze up, you will need to have it inspected by an RV HVAC technician for refrigerant leaks and repairs.

Tips on How to Keep Air Conditioners From Freezing Up

Of course, the best course of action is preventative. If you can keep your system from freezing up in the first place, you won’t have to worry about dealing with ice in your air conditioner at all.

There are several things you can (and should) do to help prevent ice build-up in the first place.

  • Change filters.
    Your air filter isn’t designed to last forever. When you purchase a filter, it will state the replacement schedule on the packaging. Most central air filters are designed for replacement every 30, 60, or 90 days. Make sure you replace them on time to keep airflow moving.
  • Annual inspections.
    Calling an HVAC technician at the start of each season is an inexpensive way to maintain your system. Most inspections last about 2 hours and will identify any problems or issues that you may have before you attempt to run the AC for the summer. If any issues are identified during the inspection, scheduling repairs is also simple and cheaper than waiting for the problem to get worse.
  • Cleaning and maintenance.
    Every 6 months, you should clean the condensing unit outside, including the coils, fans, and compressor. You should also inspect and clean the evaporator coils as needed. While this can be a tedious process, the few hours it takes to clean your system is much cheaper than repairing costly components and paying labor charges for something that could have been prevented in the first place.
  • Timely repairs.
    If a component such as a fan motor, filter, or capacitor does start to go bad, replacing it immediately instead of waiting for it to get worse is always a better choice. The longer you go trying to use a system not working at 100%, the more chance you have of wearing out other parts prematurely.

People Also Ask (FAQ)

Can I pour hot water on frozen air conditioner?

Depending on where the ice has accumulated, you can pour water on the AC. this usually only pertains to the outside unit (condenser) if the AC lines and compressor are frozen. If the ice is in the evaporator unit inside, using water to thaw may result in an overflowing drain pan, which can cause a bigger mess. You can use tap water or running water from a hose, too; you don’t need hot water to melt the ice.

What causes an evaporator coil to freeze?

There are a number of reasons evaporator coils will freeze. Low refrigerant and improper airflow are the two most likely causes. However, intermittent fans, low outdoor temperatures, and a system that won’t shut off for electrical reasons can also cause freezing.

What are the symptoms of a dirty condenser coil?

When a condenser coil is dirty, airflow is blocked. This results in the cooling of the refrigerant to stop. Pushing hot gas through the compressor prevents proper pressurization and makes the system run much colder than it should. Eventually, the condensing unit’s lack of heat exchange will cause the compressor and high pressure lines to freeze, which can quickly reach the evaporator.

How long does it take for AC to unfreeze?

The time it takes to unfreeze an AC system will depend on the extent of the freezing and where it originates. Unfortunately, we don’t have visible access to the evaporator in most cases, and if the freezing starts in the evaporator or its coils, you may not notice it until it gets bad enough to start freezing the refrigerant lines leading to the condenser unit outside. In most cases, with ambient room temperature, you can expect a full defrost in a couple of hours or less. You can speed up the process by turning the system to fan only mode, which will move warm air from the home over the coils. Otherwise, you will need to wait.

Should there be ice on AC coils?

There should never be ice anywhere on your AC system. The refrigerant running through the condenser is hot and should never be at a temperature that results in freezing. In the evaporator, the coils may show signs of slight frosting but shouldn’t ice over at all. If you notice ice on either set of coils, there is an underlying problem that should be located and repaired.

Do I need a professional to help unfreeze my AC unit?

You do not need a professional to unfreeze your AC. Simply turning the system off and waiting for the ice to melt is all it takes. However, when there is ice build-up on the coils, lines, or compressor, it is an indication that something more serious is wrong. You may need an HVAC professional to locate, diagnose and repair the underlying problem.

How do I add Freon to my AC unit without an HVAC professional?

To handle, add, remove or dispose of refrigerant (sometimes mistakenly referred to as Freon), you must hold an EPA 608 certification. Unless you work in the HVAC industry, you probably do not have this certification. If your air conditioner needs to be refilled with refrigerant, you will need to contact a licensed professional. However, low refrigerant means there is a leak in the system somewhere, which should be located and repaired before refilling the refrigerant, or your system will run low and freeze up again.

Conclusion

A frozen air conditioner is never a good thing. When working properly, an air conditioner should cool the air enough to lower the indoor temperature but not become so cold it starts to form ice.

This article explained why a system might begin to ice over and showed you how to identify, repair or fix the underlying problems causing the freezing. Hopefully, you have a better idea of how to maintain your HVAC system to prevent freezing in the first place and what to do if it does happen to you.

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