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I need help. I've been surfing the internet and have had no luck finding answers to my situation so am hoping for some guidance. I've seen other questions here answered but none that suit my issue so am hoping it can be addressed with some recommendations.

I too am trying to install (or power up) my c-wire for a new smart Honeywell Thermostat. As all sites and videos I found show, all I have to do is hook up the unused blue wire which was both available on both ends on our thermostat and the furnace board. The thing is, I installed both blue wire and still no power to the new smart thermostat. I troubleshooted a variety of things from checking the fuses, etc. I've attached a photo of the furnace board with the blue already attached as pictured The other set of two wires was to my AC unit which I kept the same.

For now, I installed back the old thermostat and it functions properly so power looks be working (blue wire remains unconnected for the old one) as pictured.

Looks correct, but I'm not an HVAC person. Any advice on other ways to troubleshoot would be appreciated.

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  • The 'C' wire gets one side of the power line to the thermostat, but there needs to be another wire back to the transformer to complete the circuit. The wiring guide should show you that wire also.
    – Some Guy
    Mar 30 '15 at 18:53
  • Are you sure the blue wire is electrically continuous from the furnace to the thermostat?
    – Tester101
    Mar 30 '15 at 19:04
  • What's the make and model of the furnace and thermostat?
    – Tester101
    Mar 30 '15 at 19:08
  • @SomeGuy The R wire is one of the transformer secondary leads, while C is the other.
    – Tester101
    Mar 30 '15 at 19:11
  • @Tester101 - I know that is the theory, but did not want to make that assumption without knowing the make and model of the thermostat. I have always considered RH and RC to be separate lines. There is a jumper shown in the old thermostat - but I was not going to guess that there was only one R terminal in the new one.
    – Some Guy
    Mar 30 '15 at 19:23
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The thermostat you posted has no common terminal (C). The blue wire that should be wired to common at the thermostat is cut off and wrapped around the harness.

You show red, white, yellow, and green wired to R,W,Y,G terminals on the thermostat, but there is no common terminal on that thermostat and the blue wire that should be wired to common is unused at the thermostat yet wired to common at the air handler.

Is there 24 volts at the air handler low voltage terminals (R and C)? You should, if not check for the door panel switch being closed. It must be closed or tape the switch closed if trying to work with the door open.

The circuit board in the air handler's circuit board should have an automotive type 3 amp fuse: it may be bad. If not and the breakers are on and proper voltage is to the furnace, then the transformer is bad.

If you switch the fan on does the fan work? If so this proves you have both high voltage and 24 volt low voltage control power available. The thermostat you show has no provision for the common side of power and looks to be battery powered. Perhaps a Maple Chase brand thermostat?

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