Why Does My AC Unit Have A Frozen Coil?

Date Published: June 27, 2017

It’s that time of year when the temperatures really start to rise. So it’s up to your trusty AC unit to work overtime and keep your home cool.

But one day you notice that it’s just not putting out the same cold air that it usually does. In fact, the air feels more like it’s at room temp. Or maybe your unit has been running all day long with no decrease in temperature. These are tell-tale symptoms that could indicate that your AC evaporator coil is frozen.

As a seasoned home inspection company, we have seen our fair share of defective HVAC systems. During our standard inspections, we measure the ambient temperature of the air coming out of the registers. If the temp drop is less than 10 degrees while we have the AC unit running, we know this could indicate a coil issue.

Sometimes the necessary repairs for a frozen coil are relatively easy, and handy homeowners can fix them on their own. However, sometimes the issues are more serious, and you will need to call an HVAC technician.

In this blog we will take a dive into all the potential scenarios that can freeze up your coil. But first, let’s take a step back to clarify just what an AC evaporator coil actually is.

Frozen evaporator coil in an AC unit

Image Sourced from: https://joesheating.ca

What Is An Evaporator Coil?

An evaporator coil is a system of U-shaped metal tubes (usually copper, aluminum or steel) that are filled with refrigerant and mounted in metal panels. They are an extremely important component of the air conditioning process. When the return air duct pulls warm air from the interior of the home, the blower then sends the air over the coil. The refrigerant inside the coil then absorbs the heat and condenses the moisture from the incoming air.

The moisture collects on the coil as water and then drains into a pan and through the condensate drain line. The newly cooled and dehumidified air is then sent back into the home through the supply ducts. Voila! You have nice cool air.

But the question still remains- why do these things freeze?

Image sourced from: https://safetyking.com/

Why Do Evaporator Coils Freeze?

Believe it or not, a frozen AC coil is one of the most common issues that HVAC technicians address – especially during the hottest times of the year.

Freezing occurs when the refrigerant in the coil is unable to absorb a sufficient amount of energy. In normal conditions, the cold refrigerant is able to absorb warm, incoming air to maintain a normal temperature.

But when that air is not flowing in consistently, the temperature of the refrigerant will drop, eventually causing the moisture on the surface of the coil to freeze over. The layer of ice on the coil further inhibits the absorption process, causing an eventual accumulation of ice.

Once the coil is frozen, and the house will simply not cool down, and further issues can occur. With ice building up and melting, the condensate drain system can overflow and leaks around the furnace can develop, resulting in other expensive damages. Also, your HVAC system will be working extra hard, increasing the strain on the machinery, which could result in additional damage to the unit.

What Issues Cause A Frozen AC Coil?

A frozen coil generally means that there is an issue with one of the following: airflow, drainage, refrigerant, or mechanical components in the unit. As stated above, the problems can range in difficulty – some are fairly manageable by the average homeowner, some will need professional attention.

Dirty Air Filter

This is generally the most common reason for a frozen coil. A furnace filter that is dirty will slow down airflow through the furnace to the evaporator coil. With the restricted airflow, the coil will not be able to receive enough warm air to effectively absorb heat. This, in turn causes the coil to get colder and eventually freeze up.

This is one of the most avoidable issues. All you have to do is stay on top of that filter! Make sure to change it out every 30 to 90 days (it varies somewhat based on location, number of occupants, pets etc.)

Dirty AC Coil

Without the proper upkeep, the coil itself will begin to accumulate dirt, dust and debris. This gradual buildup on the coil will inhibit proper heat absorption. And when that happens…you guessed it.. the coil freezes!

Experts recommend that you clean your evaporator (and condenser) coils one to two times a year. You can also, of course, hire a professional to do this job.

Condensate Drain Line Clogged

The AC condensate line is a crucial part of the air conditioning unit. It helps to drain all the excess moisture produced by the evaporator coil. If the water is backed up, its continual exposure to the cold temperature from the coil could cause it to freeze. If enough frozen water builds up in the drain pan, it could cause the coil to subsequently freeze up, as well.

This is another problem that can potentially be remedied with a good ol’ DIY method. There are plenty of great resources online that will show you a step by step process of how to clean the line.

Image sourced from: https://www.hvac.com

Issues with Mechanical Components

Like all machines, an HVAC system is comprised of multiple components that all serve a purpose and work in harmony with one another. So naturally, if one component isn’t working, it could affect another. For instance, if the blower is not functioning, the coil will not receive enough airflow; and thus it will freeze. Similarly, a malfunctioning thermostat could cause the temperature of the coil to drop too low, resulting in a freeze up.

If you suspect that you may have a mechanical issue along these lines, we recommend that you consult with a qualified HVAC technician.

Refrigerant Leak

Refrigerant is the chemical liquid that runs continuously through your coil. It is contained in a closed loop system, so its levels should not fluctuate. In the case that a leak occurs, the remaining refrigerant can absorb too much heat. This will cause the coil temp to drop too low, resulting in an icy coil.

Refrigerant is a hazardous material and should not be handled by untrained individuals. In the case of a leak, it is definitely advisable to hire a professional.

What Do I Do If My AC Coil is Frozen?

If you find that your coils are iced over, there a few steps you can take to get rid of the ice. First of all, you can turn off your system and let the ice thaw on its own, making sure to clean up the excess water with a shop vac, buckets, rags or whatever works best.

You can also turn your system to fan only mode to run warm air over the coil and quicken the thawing process. Using a hairdryer or heat gun on a low or medium (not high!) setting can quicken the thawing process, as well. Again, make sure to clean up that excess H2O.

After the ice has melted and the coil is dry, inspect your AC unit to diagnose the issue. It might be fairly simple fix, like an air filter replacement or a condensate drain line cleaning. On the other hand, you may need to get your system evaluated by a professional in order to pinpoint the issue.

Thawing a frozen evaporator coil in an AC unit

Image sourced from: https://www.myazac.com

AC Unit Maintenance

Here at Scott Home Inspection, we are strong advocates for routine maintenance! We see a number of AC unit issues that could have been avoid by some easy, regular upkeep.

Sometimes the simplest things can cause the biggest problems. Change your filter regularly to prevent costly service calls, and to keep your system running smoothly. And if your AC is not cooling well, consider a service call from an HVAC technician to clean the coil and evaluate the system performance.

Visit our Ask-Your-Inspector page for more home maintenance tips, or click here to learn more about our Scott Home Inspection Services. 

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About the Author: Chris Kimmel

Chris Kimmel worked as an Associate Home Inspector for two years, handling numerous services including sewer scope inspections, pest inspections, mold air sample testing, radon testing, and water quality testing. Chris now works with Scott Home Inspection as a Content Writing Specialist.

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