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I recently purchased a smart thermostat. Because it needed a common wire and I only had a 3 wire setup, I ran a new thermostat wire to my furnace. However, I now get only 15 volts (inconsistently, sometimes I'm getting 0 volts) between the R and C wires where I was going to connect to my new thermostat.

I then went to the furnace and tested the R and C contact and again only get 15 volts (again, at certain times I'll get nothing). The status code is blinking a code 24 for this Bryant unit which supposedly corresponds to "Secondary voltage fuse is Open - check for short circuit in secondary voltage (24vac) wiring". However, if it was a blown 3amp fuse, wouldn't I get no voltage on the contacts and not the reduced voltage that I am getting? Any thoughts on what this could be? Could it be the transformer?

zoomed in circuit board Zoomed out circuit board

  • Can you measure the voltage directly from the transformer? – JPhi1618 Aug 27 at 21:28
  • I can try. I think the blue and red wires central in the photo from my posting are coming straight from the secondary on the transformer. Can I pop those off and test across those? Or is it advised to try to test directly on the leads at the transformer (which is a little harder to access given the arrangement. – learning by doing Aug 27 at 22:27
  • Testing the end of the wires would be fine. – JPhi1618 Aug 27 at 22:59
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    While you're in testing, can you pull the fuse out and check it for continuity, just to rule it out as a cause of this issue? – ThreePhaseEel Aug 28 at 1:46
  • Went back under the house and tested the low voltage side of the transformer which showed 28 volts. tested the fuse and it was intermittent 15-16 and looked blown. Replaced the 3amp fuse and voila 28 volts at the R-C. Not sure how I was getting 15 volts with a blown fuse, but in any event the issue seems to be resolved. Thanks for the help. – learning by doing Aug 28 at 20:59

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