It should go without saying, but finding your AC filter iced up isn’t normal.
But what most people don’t realize is that it usually isn’t the air filter itself that’s causing the problem. In most cases, a frozen AC filter is caused by condensation dripping down onto the filter, then subsequently freezing up.
In this article, I’ll explain why an AC filter freezes, what causes the filter to ice over, and what you should do to fix the issue and get your AC running again.
Why does an AC air filter freeze?
A frozen air filter is almost always caused by one (or both) of these things:
- Low airflow
- Low refrigerant
But first, I want to make this clear:
Ice doesn’t form on the air filter first. What usually happens is the evaporator coil freezes first because of low airflow or low refrigerant. As more ice builds on the evaporator, it eventually spreads onto nearby components, including the air filter. That’s why a frozen air filter is usually a symptom of another HVAC problem rather than the actual cause.
Next, I’ll break down the two major causes of an AC filter freezing and what you can do about them.
1. Low airflow
Anything that reduces the amount of airflow passing through your air conditioner can cause its air filter to freeze.
When airflow is restricted, there isn’t enough warm household air flowing through the evaporator coil to keep it above freezing temperatures. As a result, the evaporator coil becomes colder and colder until humidity from your home’s air begins freezing onto the coil.
Once the evaporator coil starts icing over, airflow becomes even more restricted. This causes the air surrounding the coil to become even colder, allowing ice to spread onto nearby components such as the air filter.
In some cases, the evaporator coil becomes completely encased in ice. As the ice thaws, water drips into the drain pan. If the condensate drain line is clogged, the drain pan can overflow and drip on the air filter. Since the air filter is located near the frozen evaporator coil, the water trapped in the filter can also freeze, which creates a frozen AC filter.
Here are the most likely causes of low airflow in your AC system:
Dirty, plugged-up air filter
A dirty air filter restricts airflow through your AC. With less warm air flowing through the system, the air filter can become cold enough to freeze.
Dirty evaporator coil
A plugged-up evaporator coil makes it harder for the coil to absorb heat from the air. This can cause the coil to freeze over, causing the filter to freeze as a side effect.
Dirty blower wheel
A blower wheel covered in dust and grime moves less air. Reduced airflow can eventually cause the evaporator coil and air filter to freeze.
Bad blower motor
If the blower motor isn’t running at full speed (or isn’t running at all), ice can quickly begin forming on the air filter due to low airflow.
Blocked return air vent
Any object that blocks a return air vent can reduce the amount of air flowing through your AC system.
Closed supply registers
Closing your home’s supply vents reduces the amount of airflow through your AC system. The reduced airflow can cause ice to form on your air filter.
2. Low Refrigerant
Low refrigerant pressure lowers the temperature of your evaporator coil. Even if airflow in your system is perfectly normal, the coil can become cold enough to freeze up and ice over the air filter.
Refrigerant leak (most common)
A refrigerant leak reduces the pressure inside your AC’s evaporator coil, allowing it to become cold enough for ice to form in your system.
Improper refrigerant charge after service
If your air conditioner was recently serviced, it may have been charged with too little refrigerant.
Note: If your AC technician “topped off” the refrigerant in your AC, then there is a refrigerant leak in your system. Your AC does not consume refrigerant and should not need to be refilled with refrigerant if it is working correctly.
Restricted metering device (TXV or piston)
The metering device controls how much refrigerant enters your AC’s evaporator coil. If it becomes clogged or restricted, your AC coil and filter can become too cold and freeze over.
Restriction in the refrigerant circuit (less common)
A restriction elsewhere in the refrigerant circuit, such as a clogged filter drier or damaged refrigerant line, may reduce refrigerant flow through your system, lowering the pressure in the evaporator coil and causing ice to form on your filter.
Should I Just Replace the Frozen Air Filter in My AC?
No. Replacing a frozen air filter without addressing the underlying problem will only provide a temporary fix.
If your air filter is frozen, there is almost always another issue causing your air conditioner to freeze the air filter. In most cases, the problem is either restricted airflow or low refrigerant.
Before replacing the filter, turn off your air conditioner and allow the ice to completely thaw. Once the system has thawed, replace the air filter and restart the air conditioner.
After restarting the air conditioner, monitor it closely. If the air filter or evaporator coil begins freezing again, the problem is likely something more serious, such as a refrigerant leak or an airflow issue, and the system needs to be inspected by an HVAC technician.
