The main power, usually 220 volts, comes from a breaker and is fed into the contactor.
The contactor is controlled by the thermostat to turn on/off the unit, by letting 220-volts into the HVAC system.
The thermostat controls the contactor with 24-volts that is sent to a coil in the contactor, creating a magnetic field that pulls it closed and sends power to the unit.
The 24-volts is controlled by the thermostat setting, which turns the unit on or off depending on the temperature.
Contactors come in three styles single pole, double pole, and triple pole.
Example Single Pole Contactor on Amazon
Packard C140A 1 Pole Contactor Coil Contactor, 40 Amp, 24V
On the sides are blue and yellow wires, which is where the 24-volts comes in from the thermostat.
When the thermostat reaches the set temperature, 24-volts is sent to the contactor, which will magnetize the coil and activate it.
How To Replace a Contactor on an Air Conditioner or Heat Pump
- Power down the unit at the disconnect or breaker.
- Examine the wiring take pictures or write them down, note the L1, L2, T1, and T2 wires.
- Remove the bolts holding the old contactor in place.
- Screw the new contactor into place.
- One wire at a time, remove the wires from the old contactor to the new one.
- Power back on and test the unit.
The above is a simple breakdown of a contactor relay and how to replace a bad one.
The information will apply to Single Pole, Double Pole, and Triple Pole contactors.
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 or air conditioning relay is a straight forward simple job as long as safety is taken into account, such as turning off the power.
Replacing the wires back correctly will be the only real concern; as long as pictures or notes are taken prior to removing the relay, this should be a quick and easy job.
Have you replaced a contactor on an air conditioner or heat pump? Let us know your thoughts below.