The contactor/relay in a heat pump is what controls the power coming into the unit.
The main AC power (115/230 volts) is plugged onto one side coming from a breaker, and the units blower motor, compressor, control board and everything else plugs onto the other side.
When 24 volts is sent to the contactor a coil in the contactor creates a magnetic field that pulls it closed and sends power to the unit.
The 24 volts is controlled by the thermostat which turns the unit on or off depending on the temperature.
Contactors come in three styles single pole, double pole, and triple pole.
Here is an example Single Pole Contactor on Amazon
Packard C140A 1 Pole Contactor Coil Contactor, 40 Amp, 24V
Here is a single pole 120 volt contactor on a residential heat pump
On the sides are a blue and yellow wires which is where the 24 volts comes in and will magnetize the coil in the contactor closing it. The 24 volts is activated by thermostat.
The small plastic cover can be taken off to examine the contacts. The center can be pressed on to see how they are touching when pulled in by the coil.
Replacing a Single Pole 115 Volt Contactor on a Heat Pump
- Power down the unit at the Disconnect or Breaker.
- Examine the wiring and write them down, note the L1, L2, T1 and T2 wires. Take pictures to refer back if necessary.
- Remove the wires and remove the old contactor.
- Install the new contactor.
- Referring back to the pictures or notes plug back in the wires.
The above is a simple break down of what a contactor/relay is and how to replace it.
The same will also apply to a Double Pole and Triple Pole contactors/relays.
Some units will have the 24 volts placed only on one side or at the front. While not common some units may have a slightly different layout.
Replacing a heat pump relay is a straight forward simple job as long as safety is taken into account such as turning off the power.
Putting the wires back correctly will be the only real concern but as long as pictures or notes are taken prior to removing the relay this should be a quick and easy job.