Heat pumps use exactly the same refrigeration principle as air conditioners.  However, they have a few more parts.  One critical component is called the reversing valve.  This valve is located in the outdoor unit.  It reverses the direction of the refrigerant flow through the circuit.  These valves are typically energized when in air conditioning, which means that the unit must see a signal from the thermostat to tell it to run in air conditioning mode; otherwise, it will run in heating mode.

The “run” command is also a little different.  The heart of the control circuit is the transformer located in the air handler that converts the input high voltage (220 volts) down to 24 volts.  This control signal then sits at the thermostat waiting to be called to action.  When the thermostat sees a need for cooling (or heating, depending on the mode of operation), it sends the 24 volt control signal to the air handler to tell it to energize the blower motor.  It also sends the signal out to the heat pump.  There, the control signal energizes a magnetic relay (called the “contactor”).  When this relay closes, it sends the high voltage signal (220 volts) to the compressor and fan motor.

The heat pump will run as long as there is a call from the thermostat to do so.  In addition, it receives a second control signal from the thermostat to command it to run in cooling mode (no signal means it will run in heat).  Periodically, the heat pump (when running in heat) will have to remove accumulated ice from the outside coil.  This is called the defrost cycle.  When defrost begins, the heat pump will turn off the outside fan, but the compressor continues to run.  The reversing valve shifts so that the unit is running like an air conditioner because this causes the outside coil to warm and melt the ice.  When defrosting, the coil in the air handler in the house will be cold, so the heat pump sends a control signal back to the air handler to energize the auxiliary heat.  The auxiliary heaters are coils of wire inside the air handler that look very similar to a clothes dryer heating element.  They glow orange/red when energized and provide heat for the space while the heat pump is defrosting.

Heat pump systems also have a backup in case the heat pump is malfunctioning and needs to be repaired.  If thermostat is placed in “emergency heat” mode, the heat pump will not operate.  When the thermostat calls for heat, it commands the air handler to energize the blower and the auxiliary heating coils described above.