A heat pump is an Air Conditioning system which can also provide very efficient heating. It takes less energy to relocate heat than it does to create it. Just as a refrigeration system removes and relocates heat from a cooler or freezer, so too does a heat pump remove heat from cold outdoor air and relocate it to within a building.4 way reversing valve is used to swap the functions of the evaporator and condenser in order to change from cooling to heating mode. To avoid confusion it is common practice to call heat pump coils the "indoor" and "outdoor" coil. The diagram below shows a heat pump piping layout. Click back and forth between Buttons 2 and 3 and note how the discharge from the compressor is diverted to different coils in each mode. You can also view the 2 modes showing the refrigerant states with Buttons 4 and 5.
4 Way Reversing Valve
Here is another view of how the 4 Way Reversing Valve diverts flow; the pipe on the single stub side of the valve is always the discharge from the compressor. The discharge is diverted to the condenser, the middle stub is always suction going to the compressor, and the left over stub is always from the evaporator. The switching function is accomplished by the sliding back and forth of an internal barrel which has diverting passages. systems own high side pressure is used to ram the barrel to the desired end by a pilot duty solenoid valve. On the diagram below, click back and forth between Buttons 3 and 4 to see the barrel slide to opposite ends. Then try Buttons 5 and 6 to see the refrigerant flow being diverted.
Liquid Line Check Valves & Bi-Flo Driers
The Liquid line always carries liquid but the direction of flow reverses in each mode. Each coil has it's own metering device with a check valve piped in parallel. When refrigerant flow meets a check valve in the blocked direction it has no choice but to divert through the TXV. When refrigerant flow meets a check valve in the allowed direction it will take the route of least resistance and go through the check valve and ignore the TXV. There are TXVs that have this check valve function built right into the valve itself. The Bi-Flow filter/drier works in a similar manner. Inside a bi-flo drier there are check valves which ensure that refrigerant flow goes through the drier core in the same direction regardless of the liquid line flow direction. The drier core must experience flow in one direction only. Otherwise, if counter-flow was allowed debris that had been filtered out would unload and re-enter the system.
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.