We’ve just introduced a new interactive exercise to help you practice voltages measurements and understand contactors.

Contactors are used in HVAC systems as a means to control the voltage applied to loads. They are an electrically controlled, remote switch, used to power up motors, compressors, and sometimes the electric strip heat in residential and light commercial systems. In residential applications, a voltage of 24 volts is used to energize or turn on the contactor. In commercial applications, 120 volts or 24o volts can also be used to energize the contactor.

When voltage is applied to the solenoid coil of the contactor it creates a magnetic field that pulls the contactor contacts closed, applying line voltage through the contact(s) and energizing the circuit. When the 24 volt control signal is removed from the solenoid, the magnetic field is turned off and a spring pushes the contactor contacts open denergizing the circuit.

In this example, we will be looking at  a 24 volt contactor that supplies 208/240 volts to a compressor and a condenser fan motor on a Lennox heat pump. Refer to the schematic diagram below:

HVAC Training Schematic

Schematic diagram point 1 – the solenoid part of the contactor (K1), where the control voltage, 24 volts, is applied. The control voltage comes from the thermostat and is fed through the control board to ‘Y1 OUT’ and ‘COM’. When you look at the image of the contactor, Y1 OUT and COM are connected to the contactor via the small red wires.

Schematic diagram point 2 – the compressor contactor K1-1, meaning contact number 1 of contactor K1.  One side is connected to L1 (incoming power), the other side is connected to the compressor and fan condenser fan relay designated as K1 on the control board. The incoming power comes in the bottom of the contactor on the black wire. When the contactor is energized the contact is pulled closed, the power is passed through the contactor to the two black wires on top.  The power is then applied through the normally closed contact of K1 on the control board to the condenser fan motor and directly to the “C” terminal of the compressor.

Schematic diagram point 3 – incoming power, L2. In this particular instance, the second leg of the contactor is a bus bar and is connected to the white wire. This means half the incoming power is always supplied to the compressor and condenser fan. The power comes in on the white wire and exits via the two red wires (only one is visible).

Caution – Unless you turn off the incoming power the black and white wires are live and since there is a bus bar for half of the power, all wires are energized.


Refer to the schematic diagram while practicing your voltage measurements on the interactive exercise. Exercise one is a properly operating deenergized contactor and exercise two is a  properly operating energized contactor. Later on we will introduce some common contactor problems and see if you can figure them out.

If you are not one of our students and would like to know more about our HVAC training programs, call or email for more information.

by Ron Walker

Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.