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?