I have a window air conditioner (about 7-10 years old) it uses the cut to size foam filters. Today, I was going to remove the filter for cleaning and found what seems to be frost across the bottom area of the (I think it's refereed to as the cooling coil) it's right under the foam filter, and has metal fins.

Anyone know what might be causing the frost? I turned off the air conditioner to let the frost melt - the foam filter was actually stuck to the frost.

  • The refrigerant has probably leaked out enough to cause the frost. At some point in the future it will cease to produce cold air.
    – d.george
    Aug 4, 2018 at 11:06
  • Both of the current answers below may be correct, if the filters or coils are dirty that reduces the flow and the coils ice up. The second possibility is low on Freon and that will cause frost build up try cleaning after the unit has been off long enough for the frost to melt. If it still ices up a simple recharge is needed most window units of that age are R134a. You are supposed to be licensed to recharge AC units but if there is a port on the low side automotive r134a can be purchased at many auto parts stores . don't know how they can sell it to understand licensed folks but it is avaiable
    – Ed Beal
    Aug 4, 2018 at 22:40

2 Answers 2


It seems like a contradiction, but ice on the inside coil almost always means the air conditioner is LOW on refrigerant. Most people think freezing up would mean too much, but in the case of AC it means the opposite.

It takes a physics lesson to explain why, but in short because it’s low there is not even cooling across coil, instead too much cooling occurs in one spot and not enough across the rest of the coil. That’s an over-simplified explanation, but to the point.

At 7-10 years old, I wouldn’t try to get that unit repaired, replace it instead—the new one will be more energy efficient and save you on electricity, todays units use significantly less electricity in most cases.


It might not need replacing.

Another possibility (beside freon issues) is that the unit is just dusty and dirty. If the metal cooling fins under the filter, or the filter itself is too dirty, there can be a severe lack of airflow. The incoming air is supposed to heat those cold coils, and if there's not enough air, it can't heat it enough to dry the dew and if low enough, that dew will freeze into frost.

Hose everything out, front and back, let it dry overnight in front of a fan. If you can't hose, try this same experiment after a mere vacuuming out. Once cleaned, run it for an hour without any filter. Yes, w/o filter (but only for an hour). If under that best-case airflow situation you still see frost, you need to replace the AC. If there's no frost after an hour, you win; put the filter back in and resume normal operation.

This has worked for me with two different ACs. While with one I admittedly just bought time, it did make it the rest of summer without freezing over. I suspect it had coolant issues as well, but the newly increased airflow was enough to mitigate the issue for a while. The other one was just filthy (got from a smoker?) and it still works, 5 years after I considered ditching it and tried cleaning as a last resort.

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