HVAC Debug Update: I’m Making Progress

Determined to debug my HVAC problems, I created a list of steps to follow. I planned to verify the presence of power and control signals at the furnace control circuit and the condenser.1

Not feeling comfortable about my knowledge of how my HVAC is wired, I searched and found Thermostat Wire Color Codes and a thermostat wiring schematic.2

First, I verified the 24V transformer’s primary is getting line voltage.3 Then, checking the secondary, I found no voltage. I tried again. I pulled it out of the furnace and tried again. Still no dice. I guess I could have just measured the impedance across the secondary and discovered the transformer was bad4 and needed replaced.

Unfortunately, by the time I discovered this, the local appliance parts and electrical supply stores were closed for the week. Home Depot and Lowe’s were still open but neither has a matching transformer. Looks like it will be next week before our HVAC is working.4

I’m concerned: if I replace the transformer, is something else wrong that will kill the new one? Or did the transformer just die of old age? I guess I’ll find out.

Fortunately, the weather is getting cooler.[^]


  1. Essentially, this is rule #1 of debugging: is it plugged in? 
  2. Ya gotta love the Transonics wiki’s disclaimer/warning/license: “The following information may have errors; It is not permissible to be read by anyone who has ever met a lawyer. Use is confined to Engineers with more than 370 course hours of electronic engineering for theoretical studies.” I’m still trying to figure out how this applies to me
  3. Yes, this means the breaker was ON and the furnace cover was off. If you’re thinking of using my post as a guide to debug your HVAC system, you should read the preceding footnote. 
  4. It was shorted. 

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Author: Brent Logan

Engineer. Lawyer. WordPress geek. Longboarder. Blood donor. Photographer. More about Brent.

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