I am adding a WiFi thermostat. I have R,W,G,Y wires. Plus 2 extra coming from upstairs that go between humidifier and humidistat. So I ran an extra wire for the C wire. When I attach it to the C terminal on circuit board, it blows the 5 amp onboard fuse. The white wire that is already attached to the C terminal and the red wire on the G terminal go to the outside A/C unit. Could those cause a short when adding the new wire? I have a tester but not sure what to check. It could be a bad thermostat. How do I test for a short on the thermostat terminals?

Here are some images without the new wire connected:

circuit board wiring diagram circuit board showing transformer Thermostat wiring

  • What's the volt-ampere rating on the transformer? Does the humidifier have its own transformer, or is it powered by the furnace transformer? Is the C wire attached to the thermostat when the fuse blows?
    – Tester101
    Apr 7, 2015 at 4:52
  • The red wire for the A/C unit is attached to the Y terminal, as it should be. That wire will lead to the coil of the contactor, and the white wire will return from the contactor coil. So when Y is energized, power flows through the contactor coil. There should be no problem there.
    – Tester101
    Apr 7, 2015 at 4:59
  • I added a pic of the transforme. Looks like 40 V A. I don't see another transformer. I did more testing. I get about 27V between the C and R wires when both hooked up. But as soon as I attach the thermostat to the base plate, the fuse blows. Bad thermostat?
    – Ktwin
    Apr 8, 2015 at 3:35
  • When you attach the thermostat, is it calling for heat/cool, or is it in standby? What is the make and model of the thermostat?
    – Tester101
    Apr 8, 2015 at 10:04
  • It is in standby. Emerson Sensi.
    – Ktwin
    Apr 8, 2015 at 12:30

1 Answer 1


this turned out to be a bad thermostat. The manufacturer sent me a new one and it works great with the C wire attached. One test they had me do was attach just the C and R wires. When I attached the old thermostat, the fuse blew.

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