why the air condition take ice in the winter?

Question

when the ice tends to insulate the coil from the air why the heating is decrease and what hapen inside the coiles and compressor please gave more information ?

Answer 1

Actually you are describing a heat-pump. It has two modes of operation, cooling and heating. The cycle is reversed during the cooling mode.
During the heating mode, the heat pump takes energy from the outside air and warms the coil inside the house.
The outside coil needs to be cooler than the outside air in order for the heat pump to work
During the winter the outside air can be below freezing, and the heat pump will continue to drive the coil temperature down to try and get heat from it. (most shut down and go to emergency heat around 38 degrees F)
If the air is 40 degrees the heat pump coil is likely 25-30 degrees and that is below freezing, the small amount of humidity in the air freezes to the coil and begins to block the heat transfer area and reduces the heating ability.
The heat pump should sense this problem and go into a defrost cycle to clear the ice.
If this doesn't happen there could be a problem with the unit.

Answer 2

Note that when there is a wide temperature differential, e.g., when heating a house on a very cold winter day, it takes more work to move the same amount of heat indoors as on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases. This typically occurs around −18 °C (0 °F) outdoor temperature. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. In other words, when it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-coupled heat pump.

Answer 3

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Note that when there is a wide temperature differential, e.g., when heating a house on a very cold winter day, it takes more work to move the same amount of heat indoors as on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases. This typically occurs around −18 °C (0 °F) outdoor temperature. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. In other words, when it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-coupled heat pump.

Source(s):
http://en.wikipedia.org/wiki/Heat_pump