HVAC Training – Understanding Simple Circuits

HVAC Training

by Ron Walker

The key to reading schematics and troubleshooting electrical faults on an HVAC system is understanding simple circuits, looking at the schematic, and breaking it down into simple circuits.  All electrical systems must have several components to operate properly, I call them:

  • Power
  • Line
  • Switch
  • Load


Power - This is the voltage coming into the circuit. In residential and light commercial  HVAC systems it is either 120 volts, 240 volts, or 24 volts.

Line – Simply the wires that carry the power.

Switch – This is a non power consuming device that passes the power. In an HVAC system these can be: circuit breakers, contactors, relays, heat controlled switches, or pressure controlled switches. (There are also circuit boards to think about, but not right now)

Load - Loads are power consuming devices. In HVAC system loads are usually: fan motors, compressors, relay coils, contactor coils, transformers, or electric heating elements.

Fig2 Open Circuit


Above is a simple circuit with 115 volts (power) carried by the wires (line) to a simple light switch (switch) to a fan motor (load). Right now the fan motor is not turning. But… if we flip the switch the fan motor turns. What happens if you flip the switch on and the fan motor doesn’t turn? If you think about it, there are only four things to check, the power, the line, the switch, and the load. If any one of the four is faulty the circuit doesn’t work. If you think about it, it’s pretty simple.

The hard part is looking at a schematic like the one below and not getting intimidated. An HVAC system has several different types of switches. For example, if you look for the defrost thermostat on the schematic. It is a switch, but it is a temperature activated switch, it is normally closed. When the temperature increases the switch opens, when the temperature decreases it closes. This is an open on temperature rise switch. In this circuit, the power (R) is supplied from the circuit board and the load (DFT) is a component on the circuit board.

HVAC Classes

An HVAC technician must understand common HVAC schematic symbols and how they operate. Once you do, the schematic diagram becomes your road map to troubleshooting electrical problems. Look at the diagram, find L1 and L2, that is the incoming power (240 volts). Trace the wires (black lines) practice identifying the lines, switches, and loads how many can you find?

Have questions, want to learn more, interested in a Free Webinar on schematic reading? Email or call, I’ll be glad to help.

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Hey Ron! I got a job in Dallas with Chapman Air & Heat as a part of the service team. I have been working hard and have impressed them with all the good information I learned in your course. A good basic understanding of the fundamentals makes a great foundation. Just wanted to share my good news and say thanks again. After I get some money saved up I'll be back in touch about some more courses. Take care, Josh G.
I just graduated from a technical college in HVAC and am still confused about reading schematics. The 26:46 minute point of your video clarified something that has eluded me until now. I appreciate the pictures that go with the schematics. Thank you Ron!
Great teaching! You make it easy to understand. Much better than the class I'm taking in community college..Please keep them coming (videos) Because you explain the material well and with clarity, I can't wait to watch more. Keep them coming!