|The outdoor coil of a heat pump can ice up just like a freezer evaporator. The tendency to ice up increases with humidity and low ambient temperatures. There is fortunately a convenient method of defrosting the outdoor coil. That is to make the system run in the cooling mode. The outdoor coil becomes the condenser and the ice can be melted away. To keep the heat where it is wanted the outdoor fan motor is temporarily shut off. All this happens automatically and is controlled by a solid state defrost control board. Every so often it will check the temperatures of some thermisters (temperature sensitive resisters) and if a defrost is needed it will activate the defrost mode. Otherwise it will wait for the next trial for defrost. As the system progresses in a defrost it will get to the point where clouds of steam will billow out of the unit. New heat pump owners should be advised to expect this. Otherwise nuisance calls may be placed by the unknowing customer. They have even been known to call the fire department thinking the unit is on fire.
In the piping diagram above, place the unit in the defrost mode (Button 2). Note that the outdoor coil has hot gas entering it which is going to condense into a liquid as it melts the ice build up. There is no danger to the compressor because this liquid must travel through a TXV and evaporator before getting back to the compressor. However simulate the end of a defrost by putting the system back into heating mode. (Button 3). Note that the compressor is now directly sucking the contents from the outdoor coil (liquid) without the benefit of any TXV or evaporator to protect it. You have just seen the need for the accumulator in heat pump systems with reciprocating compresors. There is a trend to use scroll compressors in heat pumps. The manufacturers are omitting accumulators in those units because of the scrolls ability to tolerate some liquid content in the returning suction vapours.
The defrost system should get rid of all of the ice everytime it defrosts. If a predetermined amount of time expires the defrost will be terminated irregardless but there should be so little ice left that a successful and complete defrost should be easy to achieve on the subsequent defrost. Normally a complete defrost is accomplished every time. If there is a system or defrost fault ice may continue to grow out of control.