My bedroom is the hottest room in the house, and even with all the windows open and fans in them doesn't cool off as much as I would like when it's an acceptable temperature outside. And I prefer to sleep in a rather cool room since I find it easier to breath.

I have a window air conditioner that works perfectly in the summer, keeping the room nice and cool. But in the spring and fall, when it's cool outside (but not cool enough for me to be comfortable!) the AC will freeze over quickly.

What can I do to prevent it from freezing?

Are some window air conditioners better at working with lower outside temperatures? If so what would I be looking for?

  • How cool is cool outside? Sep 4 '12 at 12:45
  • This time of year the temperature ranges from about 18-26C.
    – Grant
    Sep 4 '12 at 12:50
  • For our Imperial Measurement users, that's 64-79F Sep 4 '12 at 12:57
  • And it's the outside grill - aka the heat ex changer - that's freezing over? Or the inside? Sep 4 '12 at 12:58
  • 3
    Get the coolant pressure checked. It might be a bit low which is one reason this can occur.
    – Steven
    Sep 4 '12 at 15:54

A typical AC shouldn't freeze over in that temperature range unless it's running 24x7. Try turning it onto energy save mode and adjusting the thermostat so that it turns off for at least 10-15 minutes every 2 hours.

  • Take the AC off Maximum. Give the compressor a chance to shut off so the fins can defrost - set the temperature setting at a level where you can hear the compressor shut off from time to time. Sep 4 '12 at 13:17

As you may know, an air conditioner works by transferring heat between coils.

This process depends on being able to dissipate both the hot and cold sides effectively. Most devices are rated to sustain a certain maximum temperature differential between the hot side and the cold side.

In your case, the "hot" side is cold enough where the "cold" side gets cold enough to freeze.

However, since we presume that your room isn't intentionally being kept below freezing we can also presume that the reason it freezes is because the "cold" side isn't being warmed enough by your cooler air (as cooler air will less readily release its heat to the cold coil than warmer air).

A simple solution is turn your AC's fan up. Increase the airflow across the evaporator and it should be able to cool more without freezing. If your fan is already on maximum then you could find another way to increase airflow or you could turn your temperature up.... Perhaps your unit is simply not large enough to do the job you're after.

  • 2
    Cleaning the filter, is a good way to increase air flow. Most people don't even realize there is a filter, so it never gets cleaned.
    – Tester101
    Sep 4 '12 at 15:37
  • The filter definitely gets cleaned regularly. I suspect the unit I have just isn't up to the job.
    – Grant
    Sep 4 '12 at 17:29

Instead of trying to get the air conditioner to fight with the heating system, I'd focus on trying to balance the heating system so that opening the windows is sufficient. A good HVAC technician should be able to help you out.


Turn it off, or set to fan/exhaust only and take advantage of that cool outside air.

  • 1
    The OP explicitly states that fans alone are not enough to cool the room to a comfortable temperature for him.
    – Niall C.
    Oct 23 '12 at 14:04

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