Why Do AC Lines Freeze and How to Fix Them

Believe it or not, air conditioning units are very complex HVAC systems, relying on a number of components that work together to produce cool, crisp air. When even just one of these components fails, it can cause the entire system to malfunction.

When an AC unit malfunctions, it works much less efficiently, producing room temperature or even warm air. This leads to excess energy consumption, wasted money, and uncomfortable living conditions.

One of the most common AC problems and questions asked concerns what causes AC lines to freeze. Even in climates with intense heat like Arizona, air conditioning unit lines can freeze up due to internal malfunctions. 

But what causes this, and how can you prevent it from happening? Keep reading to find out.

Why do AC lines freeze?


A frozen AC line almost always stems from a frozen evaporator coil. Evaporator coils are responsible for removing the heat from the air that passes over them. When the coils become too cold, they’ll freeze over and prevent air from passing through. This will almost certainly cause the air conditioning unit to stop working altogether.

There are several reasons why your AC lines and evaporator coils might freeze over. Many of them are simple issues to resolve. However, some causes for a frozen AC line may require professional help to resolve the problem. 

Here’s a list of some of the most common causes of a frozen evaporator coil.

Dirty air filter

If you’re asking yourself, “Why are my AC lines freezing up?”, you might be surprised to learn that one of the most common and easy-to-fix causes that often gets overlooked is a dirty air filter. 

When the air filter on your air conditioner unit becomes excessively dirty, it eventually clogs. This can restrict the flow of air over the evaporator coils, which will then cause them to become too cold and freeze over.

Leaving your air conditioner filter dirty for long stretches of time is also a common cause of mold growing inside your unit, as mold requires moisture and dirt to grow. It’s important not to let your AC unit become a comfortable home for it.

This is a simple issue to address, as simply cleaning your air filter on a regular basis will keep this problem from occurring.

Low refrigerant levels

Another common answer to “What causes AC lines to freeze?” is low refrigerant levels in your unit. 

Refrigerant is the substance responsible for absorbing the heat from the air. When refrigerant levels are too low, the unit’s evaporator coils freeze up due to not enough heat being absorbed.

Low refrigerant levels can be caused by a leak in the unit’s refrigerant lines, or simply by natural loss over the course of time. 

This can be remedied by refilling your unit’s refrigerant. However, this should always be done by a qualified HVAC technician, as refrigerant handling requires a license due to the dangers associated with this toxic substance.

The technician should be able to recharge the system with the correct amount of refrigerant, as well as accurately diagnose and repair refrigerant leaks if necessary.

Malfunctioning thermostat

Hand turning a home thermostat knob to set temperature on energy saving mode. fahrenheit units. Composite image between a photography and a 3D background.

A malfunctioning thermostat can also cause evaporator coils to freeze and may very well be the answer to the question, “Why are my AC lines freezing up?”.

If the thermostat is not working properly, it may not be able to accurately read the temperature in the room, causing the air conditioning unit to continue running when it’s not needed. This can cause the evaporator coils to become far too cold, leading to them freezing.

Often, the reason for a faulty thermostat is simply due to loose connections in the unit, or due to its power supply failing. These are simple fixes. However, it’s always a good idea to have an HVAC technician diagnose the issue, just in case.

They can replace the faulty component, or they may suggest upgrading to a smart thermostat and better model.

Malfunctioning blower

Another answer to the question of “Why are my AC lines freezing?” is that your unit may have a malfunctioning blower.

A malfunctioning blower fan can prevent air from passing over the evaporator coils. This issue can be rather serious and can be a headache to fix, so if this is the case, it’s important to turn off the air conditioning unit and allow the coils to thaw completely before contacting a qualified HVAC technician to resolve this issue.

How to prevent your AC lines from freezing up

Now that you know what causes AC lines to freeze, here’s what you can do to prevent it from happening.

Perform regular maintenance 

Regular maintenance, such as having the air conditioning unit inspected by a professional HVAC technician, can help prevent frozen evaporator coils and other common issues from occurring. 

We highly recommend having your air conditioning unit serviced by technicians at least once a year to ensure that it’s running correctly and efficiently. 

Scheduling regular maintenance with a reputable HVAC company is a great way to stay ahead of any system issues.

It’s also a great way to establish a continuous working relationship with your HVAC technician. Knowing your technician helps make the entire process run smoothly, as you’ll feel more confident in the work being done in your home. The technician will also be much more familiar with your HVAC system.

In other words, the more familiar your technician is with your HVAC system, the better and quicker they will be at diagnosing and resolving any problems you may have. No longer will you need to ask, “Why are my AC lines freezing?”.

On top of regularly scheduled maintenance, there are several steps that you can take to help prevent frozen evaporator coils. These maintenance steps include:

  • Changing the air filter regularly – Change your air filter once every 3 months to avoid this problem
  • Keeping the air ducts clean – Make sure that your air conditioner’s air ducts are clean and free from obstructions
  • Checking refrigerant levels – Have a qualified HVAC technician check your unit’s refrigerant levels and add more if needed
  • Keep your thermostat in good working order – Have your thermostat inspected and replaced if required
  • Having the blower fan checked – Have an HVAC technician inspect and replace your blower fan if necessary

Install a humidistat

Days that are higher in humidity can wreak havoc on your air conditioning unit. High humidity creates condensation, which, over time, may be too much for your air conditioner to handle.

Consider installing a humidistat to help keep moisture under control. This will help your air conditioner run more efficiently.

AC lines freezing up? Contact Autumn Air!

By following these tips and having your air conditioning unit serviced on a regular basis, you can help prevent the evaporator coils in your AC unit from freezing over. We hope this article helped you learn more about what causes AC lines to freeze and what to do if it happens to you.

If you’re experiencing issues with your air conditioning unit lines freezing over, or any other issue with your HVAC systems, it’s crucial to address these issues right away to avoid further damage to the system.

Coming up on 20 years in the industry, Autumn Air has the knowledge to diagnose any issue and has the experience to fix it in a timely manner.

If you live in the Avondale area, consider Autumn Air for all of your HVAC needs. Our team of friendly, professional HVAC technicians is always happy to help get your system back in working order. 

Don’t stress wondering why your system is malfunctioning, contact Autumn Air today!