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Finding the 24v common to connect a Nest thermostat

I have a series 3 ultra Weil-McLain gas boiler one zone heat only system. The old Tstat had only a red and white wire, no common. Adding this new Nest it needs a common, they sent me a Nest Power Connector, but I can't find the 24v common to connect it to.


  • pecmsg
    pecmsg Member Posts: 4,373
    Any thermostat is better then the nest. 

  • 109A_5
    109A_5 Member Posts: 999
    Hello @kjdiver,

    I looked at the wiring diagram for that boiler. There is no provided 'C' Terminal connection that is labeled as such in that boiler. If you really want to try the NEST, you can power it by connecting it to an independent 24 VAC transformer.
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,824
    edited September 2022
    I agree with @109A_5 that the wiring diagram for the boiler is not helpful. The diagram shows a transformer with blue and yellow 24 volt wires coming out of it. One of the wires is the "C" wire and if you can determine which one it is, you can tap into it.

    However, the transformer may not have enough power to supply both the boiler and the NEST thermostat. Also, if you're not careful, you could fry the transformer and circuit board.

    A separate power source is recommended. If it were me, I'd get a Taco SR501 and wire it thus:

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • HomerJSmith
    HomerJSmith Member Posts: 2,303
    edited September 2022
    Alan, you beat me to the punch.
    Well, you can hunt with a volt meter for the Common transformer connection in the Ucontrol, but there is no guarantee of success and it's complicated.

    So, I would recommend the use of a 1 Zone Switching Relay with a Com connection, such as a Taco SR501-4. The Nest would be connected to the SR501 and the dry relay contacts of the SR501 would be connected to the thermostat connections on the Ucontrol. Since the existing thermostat is just a switch, using the SR501 dry relay contacts to activate the Ucontrol would perform the function of the existing thermostat, which is to turn on the boiler.

    Then you could use the SR501 to power the Nest. There wouldn't be any conflict between the SR501 and the Ucontrol.
    Alan (California Radiant) Forbes
  • kjdiver
    kjdiver Member Posts: 2
    Thank you all for the feedback! It sounds like a separate transformer could be the answer.
  • HomerJSmith
    HomerJSmith Member Posts: 2,303
    edited September 2022
    Ucontrol instructions say DO NOT hook a transformer up to the thermostat line. That means that damage to the Ucontrol may occur.

    Do as Alan and I suggested which will separated the Taco SR501 transformer from the Ucontrol transformer. If you don't want to spend the money you can do the same thing as the SR501 with just a transformer and fan relay. The SR501 makes it easy,tho.

    A fan relay center will do the same thing. I'll diagram it for you if that is your choice.
    Or a separate transformer and a separate relay:

    I do think that the Honeywell T9 wifi thermostats comes with an C wire adapter, not an extra.