When it is hot outise, the evaporator is one of the most used parts of your semi truck’s HVAC system. Because of this, it’s prone to developing numerous problems, such as frost buildup. In this blog, we’ll answer one of the common questions among drivers — why is my evaporator freezing up? By doing this, you’ll be able to identify the causes and take the necessary steps to resolve them, preventing the development of more severe HVAC issues.

What Could Be Causing My Evaporator to Freeze Up?

Your evaporator can freeze up due to several reasons, including dirt buildup, low refrigerant, and a faulty thermostat switch. Let’s run through these reasons individually to help you diagnose your situation:

1. Dirty Air Filter

Your HVAC system’s air filters removes dust, pollen, and other pollutants from the air circulating in the cabin. If your air filter gets excessively dirty, it can become clogged and affect the airflow intake into your system. (Some semis trucks such as your Kenworth W900, T800, and T600 may not have come with an outside filter for your fresh air intake.)


Sample of a frozen evaporator. (Source: Choate's Air Conditioning, Heating and Plumbing)

Obstructed airflow may affect the evaporation and condensation processes happening within your AC. This will trap the cold air inside your HVAC — building frost.

2. Dirty Evaporator

Aside from dirty air filters, dirty evaporator coils also cause your AC system to develop frost. Once too much dirt and debris are sitting on your evaporator, the coils will fail to absorb heat from the air, affecting the heat exchange within your system. This will force the system to go overboard on the condensation process, which will freeze the water and refrigerant mixture instead of vaporizing it.

3. Faulty Blower Motor

A broken blower motor can inhibit the air from properly flowing through your system. Once cold air is trapped within the AC system, your evaporator will freeze. Aside from this, it will also exert extreme stress on the other parts of your HVAC system, causing a domino effect of damage within your unit.

4. Low Refrigerant

Another common reason for frost buildup is a decreased supply of refrigerant circulating within your AC system. It causes a drop in pressure within the evaporator, allowing moisture to accumulate on the coils. 

Your entire AC system is programmed to use the optimal refrigerant level. So, if your system has a low refrigerant supply, it’s likely caused by a leak. Unfortunately, checking your semi truck for refrigerant leaks may be challenging, as it can require infrared detection technology such as ultraviolet dyes. It’s best to get your truck checked by a professional when this happens.

To learn more about how an evaporator works, read our guide here.

5. Bad Thermostat Switch

Your themostat switch, also known as a freeze switch, tells the compressor when to cycle on and off. A broken thermostat switch will cause you compressor to continuosly run, not allowing your evaporator time to thaw out, therefore creating a large block of ice in your evaporator.

Here are some of the reasons behind a damaged thermostat switch:

  • Defective Unit – Like other HVAC parts, the default thermostat switch in your semi truck is mass-produced. So, it’s not impossible to get a defective one. If you have a new semi truck and are immediately experiencing HVAC problems, checking your thermostat switch is highly recommended.
  • Improper Installation – If you’re experiencing a freezing evaporator after upgrading your HVAC, there may be a problem with your thermostat installation. When this happens, it’s best to have your unit checked again by your installation service provider.

How to Fix a Frozen Evaporator

Fortunately, there are various ways to fix a frozen evaporator, such as defrosting, recharging your refrigerant, and getting premium aftermarket replacement parts. However, it’s worth noting that these solutions are just temporary ones, and you’d still have to get your evaporator checked by experts.


A semi truck’s thermostat

1. Defrost Your Compressor

Defrosting your AC is one of the best ways to eliminate frost buildup in your evaporator. Although this is usually a temporary fix, you can do this with your semi truck by turning on the fan at its highest setting while your AC is turned off. Let it run for 10 minutes. The warm airflow will melt the ice in your evaporator.

If there’s frost buildup outside the vent, you can also use a hair dryer to melt the ice faster. After doing this, avoid using your AC system for an hour or two to ensure your HVAC unit has dried up.

To learn more about how a compressor works, click here.

2. Change Air Filters

As mentioned earlier, clogged and dirty air filters prevent the AC from maintaining proper airflow. It’s highly recommended to change your cabin air filters every 30,000 miles to ensure the full functionality of your HVAC unit, including your evaporator.

3. Recharge Your Refrigerant

Leaking refrigerant will cause improper airflow and inconsistent temperatures. When these things happen, it’s best to get your refrigerant supply checked and recharged before frost appears in your vents.

4. Regularly Clean Your AC

Aside from changing your air filter, it would help if you regularly clean your HVAC unit. Using a fan blower on your AC vents at least once every two weeks is highly recommended, as it removes dirt and debris from the system. 

However, for semi trucks that are constantly exposed to road dirt and harsh weather conditions, it’s best to get professional cleaning services every six months. Doing this will save you from expensive HVAC repair and replacement services.

Get A New Thermostat Switch

If you have already identified that you have a bad thermostat switch, it’s best to replace it with a new thermostat switch


Sample of a high-quality aftermarket thermostat switch

When getting a thermostat switch, make sure it’s compatible with your entire AC system. Aside from this, ensure that they’re properly installed to minimize further damage.

Prevent a Freezing Evaporator on Your Semi Truck with a High-Quality Thermostat

A frozen evaporator can cause your HCAC system to work harder and also lead to uncomfortable rides. To ensure your AC system can maintain the optimal climate within your cabin, get a high-quality aftermarket thermostat switch now! 

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