“Why is My AC/Heat Pump Covered in Ice/Frost During Summer?”

July 16, 2014

It’s strange.

It’s hot outside, but frost is building up on your air conditioner/heat pumps inside evaporator coil and the refrigerant lines.

This is bad news because this shouldn’t be happening. 

If you allow your AC to keep running like this, the AC’s refrigerant will freeze all the way back to the compressor and destroy it. And that’s a very expensive part to replace! 

So first, turn off your air conditioner before it causes any harm to your cooling system. 

Then turn the fan setting on your thermostat from “auto” to “on” so it can defrost the coil.

Done? Good. We’ll explain what’s causing this problem.

What causes the evaporator unit to freeze

Your evaporator coil’s job is to absorb heat in your air using cold refrigerant.  And just like droplets build up on a glass of icy water on a hot day, so does condensation build up on the evaporator coil when it comes into contact with warm air.

But if the evaporator coil becomes too cold, it freezes the condensation. Then it’s all downhill from there. The frost insulates the coil from surrounding heat, causing it to ice over even more. 

Here are a few things that could cause the evaporator to become too cold.

Cause #1: Low refrigerant

When the cooling system is low on refrigerant, this causes pressure to drop lower than normal, making the coil abnormally cold. It’s like what happens when you spray an aerosol can—the pressure in the can quickly drops, gets cold and ices up.

So, when your evaporator coil gets too cold due to low pressure, the condensation on the coil quickly freezes.

Refrigerant is never “used up.” So the usual cause of low refrigerant is a refrigerant leak or an improper refrigerant charge.

Learn more about refrigerant charges.

If you do have a leak (you may hear a hissing or bubbling noise from the leak), you’ll need a AC professional to:

  1. Find the leak
  2. Recover any refrigerant that’s left
  3. Repair the leak
  4. Add the right amount of refrigerant. 

Repairing the leak may be expensive (and not worth doing on an older units). So consider buying a new central air conditioner as an option.

Learn whether you should repair or replace your air conditioner.

Cause #2: Poor airflow

The evaporator coil freezes when there’s not enough warm air blowing over it. So watch out for  anything that reduces or restricts airflow, including:

  • A dirty air filter— Blocks airflow to the coil.
  • Closed/blocked return vents—Reduces amount of air returning to the cooling system.
  • Blower malfunction—If the blower isn’t blowing enough air over the evaporator coil, the coil freezes.

Cause #3: Dirty evaporator coil

Because the evaporator coil is wet when it runs, it’s easy for dirt to stick to it. That’s bad news, because dirt insulates the coil from the warm air blowing over it. This causes the evaporator coil to get too cold and freeze over (this is only in severe cases).

You’ll need to get a professional to clean the evaporator coil if this is the problem.

You can avoid this scenario by changing the air filter when it gets dirty. The filter’s sole job is to protect your system from dirt. But if it’s dirty, it can’t do that!

What you can do now

Before calling a professional, try this.

  1. Defrost the air conditioner with the blower (set the fan setting to “on”).
  2. Change the air filter if it’s dirty.
  3. Open up your closed air vents everywhere in your home. Also, make sure no curtains or furniture are blocking the return vents.
  4. Turn the air conditioner on for 30 minutes to see if it freezes up again. 

If it freezes up again, you’ll need a professional’s help to figure out the cause of the problem.

If you live in the Kansas City area and need help with your air conditioner, contact Santa Fe Air Conditioning and Heating for help.

Santa Fe Air Conditioning and Heating serves the Kansas City area. For more information, contact us online


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