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.¹Essentially, this is rule #1 of debugging electrical devices: is it plugged in?

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.²Ya 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.

First, I verified the 24V transformer’s primary is getting line voltage.³Yes, this means the breaker was ON and the furnace cover was off. If you’re thinking of using my post as a guide to debugging your HVAC system, you should read the preceding footnote. 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.⁴It was shorted.

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 electrical devices: 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 debugging your HVAC system, you should read the preceding footnote.
• 4
  It was shorted.