What Causes An Evaporator Coil To Freeze?

Understanding the working of an air conditioner becomes even more confusing when you consider that the refrigerant lines are supposed to remain extremely cold without freezing.

How is this even possible? How can you keep refrigerant lines at 40˚F (about 4.4˚C) without the lines freezing? More importantly, how do the lines remain in perfect working condition during winter when outdoor temperatures dip below -52˚F (about -47˚C) in places such as New York?

It’s strange. Yet it happens. Indeed, most homeowners go through the entire year without any freezing issues. It’s partly what makes the AC the marvel it is.

Unfortunately, sometimes the mechanism that keeps the evaporator running fails. When that happens, you may experience dangerous freezing that threatens the life of the AC and your well-being.

What usually goes wrong? How can you know that your evaporator coil is frozen, and what can you do about it? Let’s find out.

How the AC and Evaporator Coils Works

Perhaps we should begin by understanding how evaporator coils and the AC, in general, work. Air conditioners and heat pumps cool and heat the home by transferring heat energy from one place to another.

In the summer, the appliances cool the home by extracting heat from indoor air and dumping it outside the house, resulting in lower indoor temperatures. During the winter, the heat pump extracts heat from outdoor air and dumps it inside the house, effectively raising indoor temperatures.

The evaporator coil contains a refrigerant that’s critical to the extraction and transfer of heat into or outside the house. The refrigerant runs into and out of the house between the condenser and the indoor air handler in a constant, contained loop.

In cooling mode, when the refrigerant enters the portion of the AC inside the house, its molecules expand, cooling down the line and allowing it to absorb heat from indoor air. As soon as it returns outdoors, it condenses the same molecules, causing the refrigerant line to heat up and expel the heat energy into the outdoor air. It’s much more complex. However, that’s what happens in general.

The process repeatedly happens until most of the heat energy inside the house is removed and dumped outside, leading to a significant drop in indoor temperatures.

What Causes Frozen Evaporator Coils?

It all starts when heat energy is not able to be absorbed into the refrigerant. This can happen for several reasons. However, it mostly occurs when return air, i.e., the heat-filled air inside your home, cannot get into the refrigerant lines.

The AC is a delicate self-sustaining system that relies almost exclusively on heat absorption inside or outside the house to keep the refrigerant at the desired temperature. If there’s no heat to absorb, the refrigerant gets extremely cold. If this situation persists, the portion of the line inside the condenser can become even colder, to the point of forming a thick block of ice.

Frozen evaporator coils pose a significant risk. It often makes the fans work harder to pull air through the ice-obstructed coil. It can also cause overheating. Or worse, it can dangerously overstress the AC compressor as the AC runs continuously, never cycling off.

The question then becomes – what could block the heat-laden air inside your home from entering the evaporator system? There are five main reasons?

1. Dirty air filters

As we’ve mentioned, the leading cause of frozen evaporator coils is the failure of warm indoor air to reach the coils. One reason why this may happen is dirty air filters. When your AC filters are dirty and clogged, they restrict airflow. As a result, very little air will reach the coils. Additionally, a clogged filter can allow dirt build-up on the evaporator coils, thus blocking air intake.

2. Inadequate Airflow

Inadequate airflow essentially means not enough air is flowing through the AC. Dirty air filters, as we’ve seen, can be one cause. However, more common causes include closed-off registers and malfunctioned air handlers. If there isn’t enough airflow through the air conditioner, the refrigerant won’t have enough heat to absorb to stay in liquid form.

3. Low Refrigerant

The refrigerant line is a closed loop. So, cases of low refrigerant aren’t prevalent. However, two issues can result in low refrigerant – leaks and improper refrigerant charge. In fact, one of the first signs of low refrigerant is a frozen evaporator coil. Fortunately, both are issues you can address without much trouble.

4. Blocked Condensate Lines

Condensate lines are found on the outdoor condenser, where they help drain away excess moisture from indoor air. The moisture condenses to water when exposed to the extremely low temperatures of the refrigerant.

It then flows into a drain pan. However, if there’s a problem in the drainage, such as an obstruction, the water can collect and freeze, causing the evaporator coil to freeze too.

5. A malfunctioned Thermostat

The AC thermostat is tasked with regulating temperatures and controlling how the AC responds to climate changes to keep your room cool and comfortable. A malfunctioned AC may not be up to this task, which may leave the AC running too long. In addition, overworking the AC can wear it down, potentially resulting in frozen evaporator coils.

How to Fix Frozen Evaporator Coils

You can prevent frozen evaporator coils through regular maintenance and seasonal professional AC inspections. However, despite the all that you still encounter a frozen evaporator coil, proceed as follows;

1. Turn off the AC at the thermostat

This is critical to prevent compressor failure. Then, turn the fan switch to “Fan Only” mode to blow warm air from the house over the coils to hasten the melting. Make sure you have a container to trap the melting water to prevent water damage.

2. Check the filter

We’ve established that dirty filters are one of the leading causes of frozen evaporator coils. When the filter is dirty and clogged, airflow drops below specification. Low airflow can result in a temperature drop of up to 32 degrees in the evaporator unit. It’s impossible to keep refrigerant in liquid form at extremely low temperatures. It’s best to replace the clogged filter.

3. Check the blower fan

A damaged blower fan is another common reason for low or poor airflow in air conditioners. You can test the functioning and effectiveness of the fan by switching the AC to “Fan Only” mode and observing as it runs. Can you feel the airflow? Is it strong enough? If there’s no flow or the flow is very weak, the fan may be defective or broken.

4. Call an HVAC professional

Suppose step two works and the AC resumes regular operation without the freezing problem after you replace the filter, then great. Otherwise, you need to call in a professional. Even if you’ve determined that the fan has malfunctioned, call a professional. Changing or repairing the fan on your own can lead to further damage.


In the end, the HVAC professional should be able to fix the problem with ease. Whether it’s replacing the fan, fixing the refrigerant leak, or unblocking the condensate lines, it’s light work for an experienced professional.

More importantly, you’ll want to be more careful from there on as frozen evaporator coils pose a significant danger to your family. Left unattended, the frozen copper lines can burst just like frozen water pipes burst in the winter. It can also total the compressor, necessitating expensive repairs or a brand new replacement.