HVAC Basics: Frozen Evaporator Coil

A frozen evaporator coil is the culprit behind many calls for air conditioning repair service. Yet the cause is often misdiagnosed by inexperienced technicians. Here’s what to look for, and how to determine what’s going on. 

Symptoms of a frozen evaporator coil

Here’s what the customer is probably reporting:

• The unit is running constantly without reaching the set temperature
• Warm air is coming from the registers
• Ice or frost buildup on the indoor unit 
• Water leaks 

Not sure what the evaporator coil does? Learn more about AC evaporator coils and the refrigerant cycle.  

What can cause a frozen evaporator coil? 

When you look at the evaporator coil, you’ll find a layer of frost or even a thick coating of ice. Customers sometimes make the mistake of thinking frost is normal. It’s not! The question is, why is that ice forming? Here are the main factors that cause a frozen coil:

Inadequate air flow

Air flow across the evaporator is critically important for its operation. Without enough air flow, the coil isn’t warm enough. Refrigerant inside the coil doesn’t reach boiling point and can’t do its job of absorbing heat. The coil temperature eventually gets below the freezing point, and moisture on the outside turns into frost and ice. 

This is the most common cause of an AC coil freezing up.

Reduced refrigeration 

If the system’s refrigerant charge is too low, it can’t absorb as much heat. Just like when there’s poor air flow, low refrigerant makes the coil get colder, and moisture on the outside will freeze. 

Low heat load

The weather conditions can sometimes contribute to this problem. If it’s a mild day and the temperature is not much above the set point, that creates a low heat load condition. Combined with poor airflow and/or low refrigerant, that can be enough to trigger the formation of ice on the coils. 

How to diagnose the cause of a frozen evaporator coil

Step 1: Defrost the coil 

First things first: power off the system. Or you can disconnect the compressor and run only the fan, which can facilitate the defrost.

Now you have to get rid of the ice completely (both inside and outside) without causing damage to the coil. Never try to chip off the ice! Doing that can easily cause damage. Instead, allow the evaporator coil to defrost slowly, so falling ice doesn’t cause any more system damage. Also make sure the drain pan is not clogged so you don’t end up with water leaks.

Step 2: Measure the temperature drop 

Once the system is up and running, measure the temperature drop, or the difference between the temperature of the supply air and the temperature of the return air. It will generally range from 14 degrees to about 20 degrees. But the humidity in the air makes a difference.

You’ll need to look up the expected temperature drops (with humidity levels) for the system you’re working on. 

• If the temperature drop is higher than expected, you’re dealing with an airflow problem.
• If the temperature drop is lower than expected, chances are you have a refrigerant problem. 

If you’re interested in getting into the technical details of diagnosing the cause of a frozen evaporator coil, here’s a great video.

Investigate the causes of poor airflow:

Once you have determined that there’s an airflow problem, here’s where to look for the cause:

• Clogged air filter 
• Filter that’s too restrictive for the system (such as HEPA filters)
• Dirty coil (grime insulates the coil fins and prevents air flow from reaching them)
• Blower motor or fan problems
• Duct problems: holes or blockages 
• Poor ductwork design
• Closed registers

Investigate the causes of inadequate refrigeration:

Once you have diagnosed a refrigerant problem, here’s where to look for the cause:

• Refrigerant leaks (holes or cracks in the coil that cause loss of refrigerant charge)
• Metering device problems (device that reduces the temperature and pressure of refrigerant entering the evaporator coil)

Learn more: 

Refrigerant Leak Detection

More HVAC Basics

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