The In (puts) and Out (puts) of Control Boards

thumb hammerA Control board looks at inputs and generates outputs based on the inputs it sees. That’s it.

Let’s put this in perspective-

Input= you hit your thumb with a hammer.

Output= you swear.

If the input doesn’t happen (hammer hitting thumb) the output won’t happen (swearing)

Let’s use a gas furnace for an example.

If you jumper “R” (24 volts) to “G” (fan) on the control board’s thermostat terminals, the board closes the contacts for the fan relay, turning on the fan.

The input was applying 24 volts from “R” to ”G”.

The output was the fan contact closing.


Let’s get a little more complicated.

  1. The thermostat makes “R” (24 volts) to “W” (the heat terminal).
  2. The induced draft fan contacts close, turning on the induced draft fan motor.
  3. The air flow from the inducer fan causes the air pressure switch contacts to close, telling the PCB it’s the inducer is operating.
  4. Next, the board energizes the igniter and gas valve output after a small delay.
  5. The gas ignites, causing the flame sensor to send a input back to the board.
  6. The board responds to the flame input by starting the indoor blower fan timer countdown.
  7. After the fan timer times out, the board energizes the blower.

Input- 24 volts to “W”.

Output- induced draft relay contacts close.

Input- air pressure switch closes, starting the gas valve delay timer countdown.

Output- gas valve and igniter relay closes, indoor fan timer countdown starts.

Output-the fan timer times out, the indoor blower contacts close.


Notice there’s timing involved, this is why you need a stopwatch. Without a stopwatch, timer, etc, the difference between 5, 10, 15, 30, 45, etc. seconds is a guess at best.

If any of the inputs are missing, the associated output won’t happen; instead a different output will occur.

Let’s assume the air pressure switch didn’t close in the previous example, instead of energizing the gas valve and igniter, the board would initiate a inducer failure fault code. The new sequence would look like this-

  1. Input– 24 volts to “W”.
  2. Output– induced draft relay contacts close.
  3. Input missing– air pressure switch remains open.
  4. Output– Fault code light indicates air pressure switch did not close.
  5. Sequence stops until problem is corrected.

How do you know if a control board has failed?  If the PCB is given an input, and the proper output doesn’t happen, there’s a good chance it’s failed.


Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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