Heat Pump Electrical and Refrigerant Components

Outdoor Heat pumps

One of the most obvious things in an outdoor heat pump is the reversing valve that switches directional flow of the refrigerant and vapor. The suction line and high pressure discharge line connections remain the same but a sliding valve changes where the refrigerant flows. Activating a small electrical solenoid will change the flow depending on whether the unit is in cooling or heating mode.

Both the refrigerant liquid line and the hot discharge vapor line have service valves that can be controlled with a regular ratcheting wrench to turn off the lines during service. The liquid line has a small Schrader valve and the vapor line is a ball joint valve. Which port you use to add refrigerant will change depending on the mode the unit is in. There are different ports to use when the unit is in heating mode compared to cooling mode.

A filter dryer is always installed on the liquid line between the metering device on the outdoor unit and the indoor metering device. This bi-flow filter’s job is to absorb moisture and block any debris that is traveling through the line.

Heating Mode

The thermostatic expansion valve is only active when the unit is in heating mode. Subcooled liquid comes into the TXV and comes out as a low pressure liquid that heads out to the outdoor coil. There is a check valve that allows the refrigerant to flow through the TXV without changing the pressure during cooling mode.

During heating mode, when the TXV is active, it is able to maintain the right amount of refrigerant heading to the outdoor coil by three pressures. Firstly, bulb pressure coming off of the head of the TXV from the low pressure suction line. There is also an external equalization line mounted near where the bulb is on the suction line. Finally, there is a spring at the bottom of the TXV. Both the external equalization line and the spring act as a closing force for the TXV and the bulb is an opening force that presses downwards. Between those three pressures the TXV is able to maintain the superheat across the outdoor coil during heating mode.


The compressor is a pressure increasing device, low pressure vapor comes in and high pressure vapor comes out. On the inlet of the compressor is a low pressure switch that is normally closed, typically only being used if there is high pressure when the unit is low on refrigerant.

The thermal limit switch on the discharge line is for the crankcase heater. The switch will open and close based on the temperatures indicated on the switch. This switch allows the crankcase heater to power up to heat the bottom of the compressor to warm the oil. Unit design varies and some units may have two pressure switches (high and low) and some may have a thermal limit for a high discharge temperature.

At the top of the outdoor coil, a fan functions to pull air in from the outside, across the outdoor coil and then pushes the air up and out. In heating mode, it pulls heat across the coil where the refrigerant can absorb that heat. In cooling mode, low temperature is pulled across high temperature refrigerant. As the refrigerant rejects heat into the air, the fan draws the heated air up and out.

There is a fan relay switch mounted on the defrost control board that will turn the fan off if the unit needs to defrost. The unit will monitor the temperature of both the ambient air and the coil. If frost and ice accumulate, the unit will switch into defrost mode. The fan turns off and the flow of refrigerant is reversed, essentially changing the unit from heating mode to cooling mode. To maintain the desired indoor temperature, an electric strip heater in the indoor air handler is powered on.

Technical Components

Follow the wiring diagram for the unit you are working on, but the wiring usually follows a common scheme. Blue is typically the thermostat and yellow is the compressor. Black is the output from the defrost board to the electric strip heating. Red is for the 24 volt power. The orange wire is for the reversing valve.

Two main power wires, 120 volts each for a combined 240 volts, and a ground wire power the unit. The run capacitor can have three or four tabs on the top depending on the unit but it is connected to the outdoor fan unit and compressor.

The capacitor's job is to store and release electrical energy. For a permanent split capacitor motor, it also creates an artificial phase that initiates the turn for the motor to start and allows the motor to continue to run.

A separate start capacitor connects to the start winding for a fraction of a second to provide extra electrical power for the compressor. Once the compressor motor is spinning and up to speed, it creates a back voltage that goes to the 521 relay switch to take the start capacitor out of the electrical circuit.

The opening and closing of the contactor will power the fan and compressor on and off. Powered by the 24 volt line, the contactor will close the electrical circuit to both of the power legs.

The jumble of wires can look overwhelming but it is just a matter of identifying the components and reading the wiring diagram.