In this unedited episode of HVAC School, Bryan and Nathan talk about some basic rules for circuit board diagnosis.

So, circuit boards are boards with circuits in them. They come in two types. The traditional printed variety contains switches, relays, and resistors. The other type contains silicon chips and uses logic and processing inside the boards. You may notice these in some high-end commercial systems (EMS), but we rarely see them in residential HVAC.

The diagnostic challenges come in when technicians are unfamiliar with what's on the circuit board. It helps to break it down and look at one thing at a time. Yes, there are lots of wires and relays. You may not know what it does. Take your time and get to know where everything goes and what the components are doing. Techs often misdiagnose boards because they simply don't understand how it works, and many of them don't make the effort to understand it.

It may help to look at the overall purpose of the board. For example, a defrost control simply initiates and terminates defrost. It controls the condenser fan, reversing valve, and heat strips, all of which have a function in the defrost process.

If you have a short on the board (no-load path), you will see arcing somewhere. You would most likely see melting on the board if that were the case.

If you have a board that has failed open, the switches are closed, and there is an input. However, the board doesn't travel through the time delay to bring on the contactor.

Bryan and Nathan also cover:

• Isolation diagnosis
• Best practices
• Pulse-width modulation (PWM)
• "Ghost voltages"
• Jumper wires vs. meters for diagnosis
• Blown fuses

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