Why Does My AC Keep Freezing Up? A Portland Tech Answers

If you’ve dealt with a frozen AC before, you know it can lead to warmer indoor temperatures and even prevent your AC from turning on. Needless to say, nobody wants to deal with those issues on a hot summer day.

In this blog, we’ll help you understand what causes your AC to freeze and how to fix the problem.

The main cause for your AC freezing over is refrigerant temperatures are too low. When refrigerant temperatures are low, ice can form inside the indoor air handler or on the refrigerant lines that connect the indoor and outdoor units.

So, what causes refrigerant temperatures to drop?

The following problems can lead to low refrigerant temperatures:

  • Dirty air filter
  • Blocked return vent
  • Dirty evaporator coil
  • Low refrigerant levels

Let’s look at each of these problems in more detail below.

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Problem #1: Dirty air filter

If your AC keeps freezing over, the first thing you should check is your air filter. Replacing a dirty air filter is an easy fix, which is why we recommend this step first.

A dirty air filter prevents air from blowing over the evaporator coil, which is a component that houses refrigerant and cools your home’s warm air. The refrigerant inside the evaporator coil is normally very cold—but warm air is always blowing over the coils, which prevents them from freezing over.

An evaporator coil removed from the indoor unit

However, a dirty air filter blocks warm air from blowing over the coils. This causes the coils to get very cold, resulting in ice buildup.

What to do: Check your air filter and replace it if it’s dirty. After you’ve replaced the dirty filter, go to your thermostat and turn the main control from COOL to OFF and set the fan setting to ON. Wait a few hours to let the ice thaw and then turn your AC on again. (When you turn on your AC again you can switch the fan setting to AUTO, so the fan doesn’t run non-stop.)

Clean air filter versus clogged air filter

A clean (left) vs dirty (right) air filter

Problem #2: Blocked return vent

Similar to a dirty air filter, an obstructed return vent also limits the amount of warm air blowing over the evaporator coil. Eventually, this lack of warm air can lead to ice forming on the coils.

A return vent

What to do: Make sure no furniture, drapes or other objects are placed in front of the return and supply vents to keep them unobstructed.

Problem #3: Dirty evaporator coil

Another possible cause for a frozen AC is a dirty evaporator coil.

Even though the evaporator coil is secured inside a sealed compartment, dirt and debris can still find their way into the air handler and settle on the coils. If enough dirt builds up on the evaporator coil, warm air cannot enter, which can lead to a frozen AC.

What to do: You should contact an HVAC professional to carefully clean the evaporator coil. Usually, a dirty evaporator coil will have mold growth, which should be properly removed and disinfected. Because the job is complex and the coils are delicate, it’s best to let a certified pro handle the cleaning.

Problem #4: Low refrigerant levels

If your air conditioner is low on refrigerant, then the pressure inside your AC can drop, causing the refrigerant to get very cold. (Refrigerant expands in low pressure, and expansion leads to colder temperatures.)

As we explained earlier, when refrigerant gets too cold and isn’t counterbalanced by sufficient warm air, ice can form on the evaporator coil or refrigerant lines.

So, what causes low refrigerant levels?

Most often, your AC has low refrigerant because there’s a leak somewhere in the system.

What to do: Because refrigerant is a potentially harmful substance if not handled properly, it’s best to let a professional handle any refrigerant repairs. You should contact a certified technician to find the source of the refrigerant leak and repair it.

Need an AC repair from a Portland pro?

Request appointment today

We offer fast, same-day service from NATE-certified technicians. We can quickly find what is causing your AC to freeze up and fix the problem.

For more information about what to expect when you hire us, visit our AC repair page.


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