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My girlfriends big window AC in her living room (20,000 BTU) keeps freezing up. The coils on the inside keep icing over, and she has to turn it off and let it defrost so it will work again. I thought she was just turning it off at nights to save energy, but found out that this was the problem. Any ideas what could be wrong?

EDIT:

A little information that might be helpfull, last summer it lost a lot of cooling capacity, so I took the cover off and cleaned the outside coils with coil cleaner and a hose, got about 10lbs of dirt out of the thing (it litteraly weighed less putting it back in). This year when we took it out of storage, I cleaned it again before it went back in, so everything should be nice and clean. It has been in service for roughly 6 weeks, and she said it started last tuesday. It also got a brand new filter this summer when it went back into service.

is the return block

A/Cs can over heat if the exterior radiator, the condenser,(hot) is not allowed to breath sufficiently.

The interior radiator, the evaporator, (cold) may freeze up if not allowed to breath sufficiently. Make sure it is clean. I think you said you already have. Make sure you clean not only the filter, but the evaporator as well.

Freezing will also occur if the temperature sensor on the evaporator fails. ACs work in different ways, but most window units I have seen have a visible copper 'wire' (not a wire but small enough to look like one with a bulb at one end) attached to the evaporator to sense when it is cold enough to freeze. If the evaporator gets cold enough to start freezing the sensor will at that point tell the compressor to shut down and will only allow it to cycle back on after the evaporator warms.

Make sure the copper 'wire' sensor is located in front of the evaporator and has not been pushed off to the side somewhere. This will happen on some units as the filter is slipped in and out. There should be a clip in the fins to hold the bulb close to the evaporator, but they often fail.

Hope this helps.

It may also freeze up if there is insufficient freon charge (observed in an industrial context) if everything else is working properly.

Shadow Catcher wrote:It may also freeze up if there is insufficient freon charge (observed in an industrial context) if everything else is working properly.

I second this suggestion. It is quite common for AC evaporator coils to ice up if the freon charge is low.

CD

Shadow Catcher wrote:It may also freeze up if there is insufficient freon charge (observed in an industrial context) if everything else is working properly.

Agreed, but that is true in Ac systems that measure the difference in pressure between high and low side to either vary the orifice valve or cycle the compressor.

This is a more difficult fix as most window units are sealed and access to the low side fo charging is not an option without tapping.

I was focusing on more frequent problems that are also able to be fixed without special tools or knowledge.

My last two a/c units have had a problem freezing up. One was originally owned by Abe Lincoln it was so old. I got rid of that and got a new one and it has the same problem. I keep the thermostat very low and it runs fine.

I just assumed it was because they were inexpensive units, or because they were undersized for the area I was trying to cool.

I'll have to check these out tonight. I was thinking about the freeon being low, but like stated, not easy on a window unit.

Thanks for all the replies