Frozen Evaporator Coils Troubleshooting - It was a very hot day and I got a ticket to respond to a call for no cooling. The customer said there was ice on the air conditioner. The heating and cooling service operator instructed the customer to turn the AC off so the ice could thaw before I got there to figure out the problem. It needs to be thawed before any AC troubleshooting can occur. There are two main causes why the evaporator coil will freeze and cause ice on the air conditioner. One is decreased or no airflow and the other is a low refrigerant charge.

For the first problem, you can likely repair it yourself quite easily for some of the cases. For example, on our list below, you can easily change a dirty air filter but I would not expect most people would want to change a bad blower motor. However, for the refrigeration problem, you need to call in a licensed HVAC technician to make the repair. A frozen evaporator coil was frozen for a reason and the ice only diminishes airflow even further making the air conditioner or heat pump totally useless for its intended purpose.

Frozen Evaporator Coils Troubleshooting

Dirty Ice On Air Conditioner - Causation

There are several reasons for a frozen evaporator coil for your air conditioner or heat pump system. Here are a few reasons with more to follow below: Frozen Evaporator Coil Troubleshooting Top Reasons

1. Low or restricted airflow across the evaporator coil is one of the biggest reasons for frozen evaporator coils.
   • Dirty air filters are the number one problem that causes reduced airflow across the evaporator coil
   • Too many supply vents being closed off or blocked. Check all your supply vents to make sure they are open and not blocking airflow
   • If the system was running with no air filter the evaporator coils could have loaded up with dirt or debris. The coil needs to be cleaned especially on the intake side.
   • Check the ductwork to make sure it hasn’t collapsed or is impeded.
   • A bad blower fan motor will reduce airflow causing the evaporator coil to freeze. Repair and replace the blower fan motor.
   • The condensing unit will not turn off because of an electrical issue
2. Refrigeration Problem
   • The most common problem with the refrigeration system is a low refrigerant charge. This means you have a refrigerant leak. When the systems run low on refrigerant charge the pressure in the systems drop. When the pressure drops the temperature of the refrigerant in the evaporator coil drops below freezing. This causes moisture in the air to freeze to the coil giving an ice maker rather than an air conditioner.
   • A restricted metering device can cause a frown evaporator coil
   • Restrictions in other components in the refrigeration system including filter driers or the copper lines.

For low airflow, the homeowner can possibly fix the problem depending on how serious the issue is. This problem can be because of dirty filters for the AC or heat pump. The problem is easily solved by thawing out the evaporator coil changing the dirty filter for the air conditioner or the heat pump system.

Refrigerant Pressures | Frozen Evaporator Coil Troubleshooting

AC or heat pump systems equipped with a low refrigerant pressure switch will not freeze. The low airflow condition will cause a drop in refrigerant pressure and cause the low-pressure switch to trip the compressor. The switch shuts off the compressor preventing a frozen evaporator coil until the pressure increases closing the low-pressure switch allowing the compressor to run again. People with air conditioners or heat pumps that have low-pressure switches will likely need to call a technician but only after checking the air filters. You may also be interested in our article Copper Versus Aluminum Coils.

Conditions that will cause low airflow include a bad blower motor, collapsed ductwork, and a dirty evaporator coil. All three conditions will likely require the attention of a technician. The call I responded to on that day turned out to be a dirty evaporator coil. The customer failed to keep a filter in the system. The air handler was in a closet next to the kitchen.

It turns out they kept the trash can next to the louvered door where the air handler was located. All the dust and dirt and some of the things that were thrown in the trash were found in the evaporator coil. That caused the reduced airflow and for the air conditioner evaporator to get ice on it. The frozen evaporator coil caused the system to not function as designed.

Reduced Air Flow and Ice? - Clogged Evaporator Coils Causing Ice | Frozen Evaporator Coils Troubleshooting

Why does the AC get ice on it when there is reduced airflow? The air in your home has moisture or humidity in it. Part of the process of air conditioning is to remove the moisture from the air. In air conditioning and heating engineers and engineering, technicians use a psychrometric chart to determine the properties of air.

This is the basis for calculating the size of the AC or heating system you need to for the load or demand. The moisture in the air contains heat along with the air containing heat. The heat in the moisture in the air is called Latent Heat and the heat in the air is called Sensible Heat.

Condensation | Frozen Evaporator Coils Troubleshooting

Frozen Evaporator Coils Troubleshooting - Both forms of heat need to be removed from the air and that is what your air conditioner does. It removes the latent heat by dropping the evaporator coil below the dew point. When the evaporator coil is operating below the dew point the moisture in the air condenses. When moisture condenses it turns into a liquid and needs to be drained off through condensation piping or pump.

This is why you see piping on your air conditioner that drains the water to the outside or see window units dripping water outside. It is the moisture in the air being drained outside after the evaporator coil condenses it.

The heat is also removed from the air (the sensible heat) by being absorbed into the cooler evaporator coil. The refrigerant in the evaporator absorbs the heat and carries it outside leaving you and your home feeling comfortable.

The Fix - Clogged Evaporator Coils Causing Ice | Frozen Evaporator Coils Troubleshooting

When there is a lack of airflow the refrigerant temperature drops below freezing. The moisture in the air around the evaporator then begins freezing to the coil and any refrigerant piping leaving the evaporator going to the compressor (Technically the suction line). The solution to fixing the ice problem is to fix the airflow problem. The evaporator coil on that day was completely plugged up with dust and debris.

Dirty Filters will cause bad effects on your air conditioner or heat pump

When I arrived the ice was thawed and I quickly identified the problem. Because of how dirty the evaporator coils was I had to pull the coil from the air handler requiring a major operation on the AC system.

After removing the evaporator coil I took the coil to a car wash. I sealed off the refrigeration piping to prevent water or moisture from getting inside the piping. I then completely sprayed the evaporator with coil cleaner. Then got the nozzle for the car wash and sprayed the coil clean. Being careful to angle the spray as to not damage the coil fins. All the dirt and dust and trash were cleaned from the coil and the evaporator coil re-installed.

AC Back to Normal | Frozen Evaporator Coils Troubleshooting

After it was re-installed everything was returned to normal. The air conditioner was started and it cooled the home as a normal air conditioner would cool a home. The customer was instructed as to the importance of keeping a clean filter in the air conditioner system. If you have ice on your air conditioner because of dirty evaporator coils it is likely because someone failed to maintain a good clean air filter in the air conditioner.

There could possibly be other reasons for ice on your air conditioner. It will most likely require the services of a good HVAC contractor. Before you call make sure you check the air filter. If it is dirty change it and thaw out the system by leaving the fan on. Leave the air conditioner off for several hours. Evaporator Coils Dirty and Ice On Air Conditioner go hand in hand so make sure your evaporator coils are clean! It is important to maintain the design pressures of airflow through the air handler and the evaporator coil.

The chart above shows the design air pressures across the evaporator coil. When airflow is restricted across the evaporator coil the amount of air pressure needed based on design is restricted. This causes a drop in pressure and temperature of the refrigeration which results in a frozen evaporator coil.

How to Unfreeze the Air Conditioner | Frozen Evaporator Coils Troubleshooting

If you have a frozen evaporator coil or you find ice on the inside unit or the outside unit or piping then the best thing you can do is to shut the air conditioner off. By leaving it running you could cause damage to internal components of the air conditioner. The damage occurs from liquid slugging in the compressor. Compressors are designed to only compress vapor and not liquid refrigerant.

When you have a frozen evaporator coil there is a very good possibility the refrigerant is not vaporizing in the evaporator as it is designed to do. The next stop after the evaporator for the refrigerant is the compressor. Some compressors can take some slugging but long-term slugging will likely damage the compressor. Changing an air conditioner compressor is an expensive proposition.

If you have a heat pump you can turn it to heating mode and this will melt the ice from the heat pump. However, be aware that the melting may happen fast and this will overwhelm the drain. Make sure you keep an eye on it so you don’t cause water damage from the rapid defrosting of the coil. Even after you have defrosted or melted the ice from the air conditioner or heat pump you need to call a professional to check the unit. That is the best way to defrost a frozen evaporator coil.

Conclusion | Frozen Evaporator Coils Troubleshooting

If the ice on the air conditioner was the result of a dirty evaporator coil then the coil will have to be cleaned before the system is put back into service. Other things should be checked once the air conditioner is running normally again. These checks make sure the problem does not reoccur and the system will be reliable for comfort reasons.

You can see from the list some of the problems with ice forming on your air conditioner are a result of poor airflow. The evaporator coil is designed for a specific amount of airflow. Without the designed amount of flow, the system will have problems and a frozen evaporator coil can occur.

A dirty evaporator coil that ices up is in the airflow category. The dirt blocks the airflow and the ability of the evaporator coil to absorb heat from the air. By checking and changing the air filter on a regular basis you can keep the evaporator coil clean preventing the problem of your air conditioner from icing up.

Frozen Evaporator Coils Troubleshooting

Resource: The Home Comfort Book: The ultimate guide to creating a comfortable, healthy, long lasting, and efficient home