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How to run a c-wire to new ecobee thermostat

Hi all, I’m installing a new smart thermostat to replace an older one for my bedrooms that only have heating connected to it. My boiler didnt have a c-wire to the old thermostat, so I tried to connect the 3rd wire to the c-wire labeled on my Weil Mclain cga gold boiler. The thermostat wont turn on, and when I check the voltage at the boiler from the c-wire it reads 17ac. Any advice? 


  • Dave T_2
    Dave T_2 Member Posts: 64
    Are you sure the red and white thermostat wires are connected to the correct black wires? They may not be labled R and W. The new thermostat need the constant power that R has, along with the common the C provides. If you cannot tell, simply swap the red and white wires and test. FYI, you should measure 21 to 29 ac volts at the red and white wires at the thermostat when the connections are correct. Good luck
  • Clintvass
    Clintvass Member Posts: 2
    The boiler also feeds off of this. Should i be using the C side of this and split it to the new thermostat? 
  • Steamhead
    Steamhead Member Posts: 16,835
    No. That transformer looks like it was installed to operate the motor on an old-school clock-thermostat. I usually remove such transformers.

    If you're referring to the C wire that goes to the gas valve, that's actually not the "common" wire. The "A" terminal serves as the common terminal, you see it goes back to the other side of the boiler's transformer. Check the voltage between that A terminal and the thermostat leads. The one that reads 24-volts to the A terminal is the "hot" wire to the thermostat- usually this wire is Red. The other thermostat terminal is the switched 24-volt line that starts and stops the boiler, usually a white wire. The third wire goes to the A terminal, that's the common.
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