So Ive been trying to install a new Honeywell wi-fi thermostat. I have been unsuccessful in any event. I have a third wire unused at every thermostat in my home which is black. This black wire tuns back to my Honeywell relay R8888A/B.I do not know how to get power from the relay board and have spent numerous hours trying to rewire the black wire to get 24vac to it. If someone could please help me out!! Thanksenter image description hereenter image description hereenter image description hereenter image description hereenter image description here

  • Do you have a voltmeter? What is the voltage between the red and white wires, because I suspect its close to zero and not 24v. What about the voltage between one of the red wires and the yellow wire coming from the transformer on the bottom left?
    – JPhi1618
    Nov 16 '21 at 15:43
  • What model is your new thermostat, and are you planning to install these thermostats on all four zones, or just one? Nov 17 '21 at 0:19

Connect your thermostat's C wire to the C terminal labed XFMR C. This worked for me using an Emerson Sensi Thermostat. Using an AC volt meter you can measure 24 volts from the thermostat RH wire and the transformer C terminal during a call for heat. This will provide power to the thermostat at all times.

Photo shows the green wire spliced into the thermostat C wire and connected to the XFMR C terminal. picture of board

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    While this may work, Answers are expected to have supporting information or documentation. This would be a much better Answer if you explained why you think this is the solution, with diagrams or manufacturer specs to support that. Dec 29 '21 at 14:47
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    I think that this is correct. But to put it a bit more clearly: There is a group of two terminals next to the green light. One has a yellow and a black wire attached. Attach the black wire that goes to your thermostat to this wire, and attach the other end to the C terminal on your thermostat.
    – gbronner
    Jan 5 at 2:47

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