Frozen AC Filter – What Causes It And How To Fix It

Frozen air conditioner (AC) filters refer to frost in the unit because of a lack of airflow. Insufficient refrigerant, dirty coils, a clogged filter, stuck expansion valve or a collapsed duct can all lead to a frozen AC filter.

Nearly 250 million households across America use an air conditioning (AC) system. A frozen AC filter can cause an entire AC to malfunction, preventing the unit from cooling your home efficiently.

In this article, we explore all the possible reasons for a frozen AC/HVAC filter and dig into the diagnostic properties, troubleshooting as well as presenting preventive maintenance measures.

How an Air Conditioner works

Air conditioners (AC) are electrical appliances that cool a room or whole house. A lot of factors can contribute to a unit malfunctioning, so it’s important to understand the components and proper working of the air conditioning system.

The following points will give you a basic overview of how an AC should work:

  • The primary components of the unit are an evaporator coil, condenser, compressor, refrigerant, and filter.
  • The system draws in warm air and runs it through the filter. Following this, the now warm filtered air comes into contact with the refrigerant. The refrigerant absorbs the air and cools it down.
  • Afterwards, the evaporator coil converts the liquid chemical (refrigerant) into gas. This cool air is then blown out of the system.
  • This is where the compressor and condenser come into play. The compressor raises the temperature of the refrigerant and the condenser, where the heat is released into the outdoor air, cools the refrigerant down.

This is how an AC uses airflow to complete a cycle to cool down a room. Each part of the unit is crucial, and a problem in even one part can prevent the correct working of the entire cooling system.

If you are concerned your AC compressor isn’t working properly, Plentiful Air has a complete troubleshooting guide available here.

6 Reasons Why Your Air Conditioner Filter is Frozen

To determine if your AC filter is frozen check the following signs:

  • Frost evident from the exterior part of the AC unit. The refrigerant pipe, as well as the outside wall of the house, show ice if the unit is frozen.
  • AC is not cooling the room.
  • Water droplets on the surface of the interior portion of the AC and on the exterior.

To confirm the issue, you can disassemble the panel and locate the evaporator coil in the indoor unit to see if it has frost.

Here are all the reasons behind a frozen AC filter:

1. Leaking Refrigerant

Refrigerant is a liquid chemical that is the most integral component of a working air conditioner unit.

The refrigerant is a low-pressure gas, that regulates the overall pressure of the system of the coils. After absorbing heat, it runs through a condenser and compressor where more pressure is applied to it for the cooling process.

If the AC unit has insufficient refrigerant, the pressure in the system drops. The low pressure prevents the absorption and transfer of heat, allowing the AC filter to freeze.

An AC does not consume refrigerant, it simply cycles it. Therefore, if your AC is running out of refrigerant, it is because there are holes or cracks in the unit allowing it to escape.

The primary reason for holes is corrosion in the metal coils as well as in the tube walls.


Since the refrigerant is leaking out of the system, it is important to repair the cracks.

Usually, an aluminum flux weld is filled into a broken AC to restore its shape.

Since corrosion causes the problem, regular maintenance of metal parts of the system can help prevent the issue.

According to the US Environmental Protection Agency, you should get an HVAC expert to recharge the AC unit as mishandling of Freon (a common type of refrigerant) can pose a hazard to both personal and environmental health.

2. Dirty Coil

Although the filter is in place to clean the air of contaminants before letting it pass, some amount of dust can still accumulate on the evaporator coil. Since the air comes into contact with the coil and the refrigerant, any debris on the coil can constrict adequate airflow.

This hampers the effective absorption of the hot air as well as the evaporation of the refrigerant.


Regular maintenance to clean the coil is the only way to prevent a dirty or corroded coil. Depending on the condition, the coil may even need replacement.

You need to check in with an HVAC professional for AC maintenance at least once a year.

3. Clogged Filter

A filter is present on the AC’s return side. It traps the pollutants from the air. Over time, the dust build-up can block the filter, preventing efficient sanitization.

A clogged filter causes the AC or HVAC system to overheat. Overheating naturally sends the fan under stress which impacts the airflow and eventually the entire unit starts malfunctioning.

Filtration of the air is paramount for the safety of other components of the unit. As the contaminants interfere with the AC components and prevent them from working properly, the filters end up freezing.


A damaged or clogged filter needs to be replaced for the system to resume its normal working.

Most AC/HVAC companies recommend changing an AC filter every 90 days, but depending on the air quality, you may need to replace it after 45 days to ensure maximum efficiency.

If your filter is just wet and not frozen (with ice), Plentiful Air has more information to help you troubleshoot your problem available here.

4. Stuck or Clogged Expansion Valve

The expansion valve is located in the line between the condenser and evaporator coil. It is important for the expansion and evaporation of the refrigerant.

It reduces pressure from the warm refrigerant converting it from liquid to vapor.

If the valve becomes clogged or is unable to open, it won’t release pressure off the refrigerant, which will prevent the cooling.

Conversely, if the valve gets stuck open, it will not regulate the amount of refrigerant passing into the compressor, which will lead to overheating.

Following are the signs to look for to confirm a stuck expansion valve is the cause of a frozen AC filter:

  • Frost on vent
  • The compressor’s light is always on
  • Warm room


Locating the expansion valve and detaching it from the unit can be difficult for a layman. So if you suspect a dysfunctional expansion valve, consult an HVAC expert to look into the problem.

5. Blocked Condensation Drain

Humidity, or water vapor from the air, enters the AC unit along with the air. The condensation duct prevents moisture build-up in the AC/HVAC system.

When the air comes into contact with the refrigerant, the moisture condensates and is led away from the refrigerant by the condensation duct. This is important as moisture build-up can lead to the development of mold and mildew in the unit.

If the condensation duct gets blocked by debris due to poor filtration, it obstructs the passage of moisture, which damages the system. The prevalent humidity causes the filter and the evaporator coil to freeze.

This is usually indicated by the presence of a mold-like smell and frozen coils.


Regularly clean and replace your air filters to prevent the problem. If the moisture builds up in the unit, call in an HVAC expert to unclog the duct.

Unplug the appliance for at least 24 hours allowing the ice to melt.

You can also place a dehumidifier in the room to soak in the humidity from the room, so that warm but dry air enters the AC unit.

6. Faulty Blower Fan

The blower fan draws air into the system and pushes it towards the evaporator coil. Any fault in the rotation of the blower fan impairs the airflow.

As the inflow of hot air decreases, the temperature in the unit falls, as a result of which, the AC filter freezes.

Two factors can be responsible for a damaged blower fan. Either an obstruction due to improper filtration gets stuck in the way of its rotation, or there is a fault in the motor affecting the fan.

Whatever the cause, a faulty blower fan needs prompt troubleshooting, as it also puts the compressor at risk.


Turn the AC off and inspect the fan for any debris accumulation.

  • Manually rotate the fan blades and see if they move easily.
  • In case of a build-up, clean the fan and turn the appliance on.

If the problem stems from the motor, it will probably need replacement. Consult a technician to confirm and resolve the issue. Make sure to not plug in the system till the problem resolves to ensure there is no secondary damage.

Frequently Asked Questions

Here are some of the common questions asked about frozen AC units:

Can a dirty filter cause an AC to freeze?

A dirty filter impairs the airflow and causes an air conditioner (AC) to freeze. Poor filtration, and dust and debris build-up in the AC components including the fan, condensation duct, evaporator coil and compressor, lead to a frozen and malfunctioning AC unit.

AC systems have an air filter that traps the pollutants from the air. If not replaced timely, it can accumulate a lot of dust that blocks the pores and prevents any further filtration.

This puts the unit under stress, causing overheating, which not only damages the system but also results in the emission of hot and impure air into the room.

A dirty filter does not only directly impair the functioning of the AC but indirectly also damages other components. It can:

  • Obstruct the blower fan, impairing the efficient flow of air.
  • Dust can build up on the coil, preventing the air from passing through.
  • It can constrict the condensation duct, leading to water damage.

Therefore, it is important to change the filter on time, i.e. every 90 days.

How Long Should I Leave My AC Off When Frozen?

Air conditioner (AC) units should be switched off and left unplugged to allow any frost to completely melt. The time depends on the amount of ice build-up and can vary from an hour to a day. However, a minimum of 24 hours is advisable.

Since ice build-up prevents an AC from working, you will need to turn off the appliance to let the ice completely melt and defrost the AC.

Sometimes just leaving the unit unplugged for a while is sufficient for it to resume its normal functioning. It also allows the room temperature air to blow on the unit, which helps fasten the melting process.

The frost can be caused by a whole range of reasons. Therefore, the time you need to leave the system off varies. To stay on the safe side, we recommend not using the AC for 24 hours to ensure there is no ice left inside.

Remember, whenever you face a problem, it is better to turn the appliance off and check if it’s working before calling an HVAC expert.

Can You Pour Hot Water On a Frozen AC Unit?

It is safe to pour hot (but not boiling) water on a frozen air conditioner (AC) unit to defrost it. Pour warm or hot water in a stream onto an unplugged unit. Allow it to dry completely before reconnecting the AC to a power source.

It is safe to pour down hot water to melt the ice down when posed with a frozen AC unit. Be sure to use hot water as room temperature water won’t do the task as efficiently. Remember to unplug the unit before pouring the water onto it.

Can Your AC Freeze In The Summer?

Air Conditioner (AC) units can freeze in the summer because the refrigerant pressure changes due to the prevalence of warm air. Setting your temperature too low, relative to the outside temperature, can also overload the AC and lead it to freezing.

In summers, an AC draws in warm air that is low on pressure. This causes a drop in the pressure of the refrigerant, which can result in a frozen unit. To prevent the problem, it is better to schedule the AC use and not operate it constantly throughout the day.

Alternatively, an AC set on a temperature that is too low causes the unit to overwork. The cooling system cannot cope and ice can begin to form throughout the unit. Never set your temperature more than 20 degrees lower than the outside temperature.

AC units also begin to fail and freeze up if you attempt to operate them when the outside temperature is warmer than 100°F.


If you are looking for a solution to a frozen AC unit, we hope this article proves helpful. AC units can malfunction due to various issues, including poor filtration, improper drainage of moisture, dirty coils, faulty blower fan, etc.

Remember to consult an HVAC expert for technical issues, as handling them yourself can exacerbate the problem.

Theresa Orr

Theresa Orr holds a PhD in Earth Science and specializes in determining past climates from rocks using geochemistry. Her passion for clean water, soil and air drives her to provide easy to understand information for everyone to read.

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