It is not uncommon for the air conditioner to freeze. When you find the air conditioner freezing up, it is a sign the AC isn't working. The AC unit can freeze in the hot summer weather or on any given day. And when it happens, it can be frustrating.
In some cases, it can be the indoor (evaporator coil) that freezes. And in other cases, if you have a heat pump. It can be the outdoor unit (condensing coil) that freezes.
This guide covers everything you need to know about a frozen AC unit. Such as why the AC freezes up. How to avoid a frozen AC unit? The potential damage a frozen AC unit can have on the HVAC system. And how to fix an air conditioner that freezes up.
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The Simple Science
The air conditioning system is a giant refrigerator. It uses the same concept as a kitchen fridge or freezer, except at a much larger scale.
The main components of an air conditioning system are a compressor, two coils (condensing and evaporating), an expansion valve, a blower fan, and a refrigerant.
If the air conditioner isn't working within the manufactures specifications. It can prevent the air conditioning system from working right. And you may find yourself with a frozen AC unit.
Depending on the type of HVAC system, the indoor (evaporator coil) can freeze, the outside unit, known as the condenser, can freeze, or both. In a central mini-split system, the indoor coil can freeze. And with a heat pump, both coils can freeze.
The system freezes because the refrigerant levels in the coil are below freezing. It is important to note that it is not the refrigerant that freezes. It is the condensation on the AC unit components that freezes. This can happen on the copper AC refrigerant lines. The indoor (evaporator) coil. Or outdoor (condenser) coil.
Why does an Air Conditioner Freeze Up?
Two main reasons cause an AC system to freeze. They are:
Below we talk about why and how these two things cause the air conditioning unit to freeze up. And the damaging effects this can have on the HVAC system.
AC Freezes up because lack of airflow
Lack of airflow will cause your system to freeze. This happens because the evaporator coil needs to absorb heat from the airflow. The removal of heat from the airflow is what cools the air coming into your home.
When there is a lack of airflow, the AC unit's evaporator coil won't absorb the heat. The heat absorption raises the coil's temperature and keeps it above freezing.
A frozen coil can lead to other problems with the HVAC system. Such as damage to the compressor and a bad AC capacitor.
This happens because the refrigerant cannot absorb heat from the airflow, which prohibits it from turning into a gas. Then the refrigerant makes its way back to the compressor as a liquid.
Because liquid cannot be compressed, it can damage the compressor. Or cause it to work under increased strain.
In some cases, you may find yourself with a freezing compressor.
Static pressure is a metric to determine airflow in an HVAC system. When the system has high static pressure. It means there is a lack of airflow volume. Common reasons for high static pressure are poor duct sizing, dirty air filters, or restricted vents.
A faulty blower fan motor or electronic failures can also cause a lack of airflow in an HVAC system. For example, a variable or two-stage system must communicate with the blower fan. It lets the system know at what capacity to run. The system can be deprived of airflow if the two components aren't in sync.
Connect with an HVAC pro. They will check if the system is freezing due to a lack of airflow, a faulty blower motor, or electronic failures.
AC Freezes because it is low on refrigerant
Air conditioning systems use refrigerant to cool a home. You can learn more about how refrigeration works. In short, it uses a compressor to compress the refrigerant. Two coils transfer heat to and from the refrigerant. And a metering device controls the refrigerant flow.
When the refrigerant level of the air conditioning unit is low, you can find yourself with a frozen AC unit. In most cases, the indoor evaporator coil freezes. This can happen in warm or hot weather in the summer when the AC is on. And other times, when the system is a heat pump, the AC unit outside freezes. This can happen when the heater is on.
A system low on refrigerant means the HVAC system has low pressures. The lower the pressure, the lower the temperature of the refrigerant. (Think of an aerosol spray can that gets cold when you use it).
When the AC unit is low on refrigerant, the refrigerant temperatures will drop. When it drops below the freezing point of water, any condensation build-up on the coil will freeze. It is important to note that it is not the refrigerant that freezes but the condensation. It is the humidity in the air condensates on the coil. And then freezes.
AC Unit freezing because a faulty expansion valve
Another reason the air conditioning system freezes is a faulty expansion valve. The expansion valve regulates refrigerant flow to the evaporator coil.
Its purpose is to reduce the pressure of the refrigerant in a controlled way. The loss in pressure is what brings the refrigerant temperature to near freezing.
A faulty expansion valve won't regulate this pressure drop, which lowers the coil's temperatures and causes the air conditioning system to freeze.
Frozen AC coils and the potential damage to the HVAC system
Catching a frozen AC coil early is vital. When the air conditioner freezes up, it prevents the refrigerant from transferring heat—affecting the refrigeration cycle. What happens is that the refrigerant won't transfer from a liquid to a gas state. When this happens, you risk having the refrigerant flood back into your compressor. This means that refrigerant in the liquid state is making its way back to the compressor. And since liquid cannot be compressed, it damages the AC compressor.
*Refrigerant levels that are too high can also damage the HVAC systems components, such as the compressor or AC capacitor. Learn more about the refrigeration cycle to understand why.
Contact an HVAC professional if the air conditioning unit is freezing. An HVAC pro will be able to diagnose the source of the problem.
How to Fix a Frozen AC system
Now that you know what causes an AC system to freeze, you may wonder how to fix it. To start, we recommend that you do not operate the AC is frozen. Doing so can cause damage to the system.
First, you want to remove the ice build-up. The fastest way to do this is by turning on the heater (don't do this if you have a heat pump). If you have a gas furnace, the heat will melt the ice.
You also want to ensure that the ac drain line is clear. As the ice melts, it should drain out of the AC condensate drain line. If the ice build-up is significant, some water may leak out of the system.
If you have time, it is recommended to let the ambient air melt the ice. This will reduce the amount of water that escapes the drainage system. If water escapes the drainage system, it will appear as if the furnace is leaking water. Be careful to avoid water damage in the home.
Once the ice build-up is gone, see if anything limits the airflow in the system. It can be return air grilles (vents) that are blocked or even a dirty air filter.
It can be a different, more severe issue if nothing restricts airflow. Such as undersized ducting, dirty coils, or a problem in the refrigerant system.
Contact an HVAC professional if you think it is one of the more severe problems.
If the source of the problem is undersized ducting, you may need to increase the duct sizes or add more. Cleaning a restricted or dirty AC coil involves opening up the coil housing. And adjusting refrigerant pressures should only be done by an EPA-certified HVAC professional.
Whatever the cause, DO NOT run the air conditioner if the AC coil is frozen. Doing so can cause damage to the system.
It can be frustrating to discover that the air conditioner coils have frozen. It can be the air conditioner coil inside that freezes. Or, if you have a heat pump, the air conditioner coil outside can freeze.
In this guide, we learned everything you need to know about a frozen HVAC system. Such as the science of why the AC freezes. What causes the AC coils to freeze. And how to fix a frozen HVAC system.
In some cases, fixing a frozen AC system is as easy as an air filter change. And in some cases, it requires the help of an HVAC professional. The HVAC unit must defrost and be ice-free for an HVAC pro to diagnose the HVAC system.
If you have any questions, contact us. We are here to help.