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I recently purchased a Honeywell wifi thermostat and tried to hook it up as a replacement for an existing battery powered unit. I have a typical forced air HVAC setup with gas furnace (no heat pump), and I have replaced several thermostats in the past at other houses on very similar systems and never had any issues.

In this case, when I removed the original battery powered unit, the C wire was tucked behind the wall, so I manually connected it back up to the wall plate and inserted the new thermostat, but it would not power on. Then I tried putting the battery powered unit back in (removing its batteries, the manual says this will work), and still that one would not power up. I got out my volt meter and measured 20 volts across R and C, and 27 volts across R and G. I put the old thermostat back in with the batteries and that unit is still working fine.

Any idea why I am not able to power either thermostat when using the C wire, but I can measure voltage across it?

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    Is the C wire hooked up at the furnace? What's the make and model of the furnace?
    – Tester101
    May 5, 2016 at 16:19
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    You might still measure voltage across R&C even if C is not connected back at the furnace. Try measuring R&C with a thermostat connected.
    – brhans
    May 5, 2016 at 16:30
  • Can you post a photo of the thermostat wiring at both ends? May 5, 2016 at 22:20
  • volt meters will give answers that are not useful. First you need to figure out if there's any power available between the C and R wire. connect a 24V 5W lamp (24V automotive parking lamp) if it doesn't light brightly the other end is not connected to the right place.
    – Jasen
    Jul 28, 2022 at 0:45

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I have exactly the same issue. R+C at thermostat measured 20 volts and R+C at circuit board is 28 volts. Also the blue wire is not connected to the circuit board in the furnace.

After checking all the wire connections and the circuit board inside the furnace I connected the loose blue wire to the c terminal on circuit and received 28 volts at the thermostat with R+C.

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  • Did this actually solve the problem and are you recommending to others that they do this too, or are you still having problems and asking for help? It's very unclear to me. If this did solve the problem, please edit your answer to make this more clear. If you're having a similar problem, please ask a new question, as there is no place for anyone to answer a question when it's asked in the "Your Answer" box.
    – FreeMan
    Jul 27, 2022 at 11:55

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