Why do coils freeze on a window air conditioner?

Our window air conditioner doesn't blow air hard enough to cool the room and when I opened the front, the coils and the compressor was frosted. I turned it off and it defrosted and worked better, but then froze again. Any ideas of what to do so I don't have to buy a new one?

Answers:
The unit freezes for one of three reasons:

1. It is low on freon thus chilling the coil below its normal operating temperature of 39 degrees.

2. The air flow across it is too cold causing further chilling and freezing. Usually a problem with dehumidifiers not AC.

3 Not enough air flowing across the coil, usually caused by excessive dirt in the machine or a blocked filter thus allowing the coil to drop to below 32 degrees..

Other answers:
Too much moisture
Too much moisture
Could be low on freon, or the fan is giving out.
too much moisture in the air i suppose, you can always call a repair person and mention this . or put it in a websearch and see what comes up.
With a high dew point this will happen to all air conditioners. The coil is called a condenser coil for a reason. It is designed to draw the moisture from the room. when the dew point reaches 70 or better on a hot day the saturation in ambient air is such that the moisture cannot evaporate and freezes on the coils. On really hot days, a way to get around this is to by a cheap pin style plug in timer and set it to shut off for 5 minutes every hour.
It is low on freon, have a ac man add some.
mine does that when i have it on constantly here's what you do its on my manual first set the temp on the highest meaning like warm , let it run for about 15 mins and the ice will slowly melt it's normal it happens to me every once in a while no biggy. then put it back to how ever you had it .
the freezing effect is caused by water (condensate) stucked in the fins of the tubings because of very fine dust accumulation. have it cleaned by an a/c technician. it will be as brand new again.

a freon recharging may also be needed.
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