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 - 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.
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.
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.