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Nest thermostat misfiring steam boiler

I'm posting to inquire if anyone had the same problem as me. I have a 3rd generation Nest that I would like to use to control a high-velocity forced air system for AC (wires Y1, G, Rc, and C) and a Burnham steam boiler for heating (wires W1 and Rh). When I only connect the AC system, all is good. The Nest uses the C wire to power itself. I measured the that C wire provides 28 VAC with a multimeter.

When I also connect the steam boiler, the Nest does not work properly. It intermittently closes the loop of the boiler every few seconds, which makes the relay click. Obviously, the Nest has decided it wants to steal power from the steam boiler even though it has a live C wire.

My plumber had suggested that maybe the wiring between the steam boiler and the thermostat was defective. I did a continuity testing and also have been running this winter with an older Honeywell programmable thermostat (connected only to the heat - wires W1 and Rh). It fires the boiler when it should and keeps it shut when no heat is called.

Has anyone encountered this similar, and rather weird, problem?


  • Big Ed_4
    Big Ed_4 Member Posts: 2,805
    Jump out Rh and W , if it runs find call up Nest

    There was an error rendering this rich post.

  • kjl520
    kjl520 Member Posts: 10
    Well... First, you should not have power on C. That is the Common/Neutral wire. The C wire should be connected to the common side of the AC transformer or if you have a C terminal on your Spacepak/Unico. You would then connect the Y and G terminals to the AC. Use the (hopefully) Red wire from the boiler to Rh and the (hopefully) white wire from the boiler to W. If the Nest doesn't see the Red or White wire then switch them around. But the main thing is that the power circuit comes from the AC, not the boiler. I connect Nest (easy) and EcoBee's (Harder) all day like this. I work for the largest installer of SpacePak in the world. I'm just injured and am in the office for a few weeks. :) Good luck!
  • Ironman
    Ironman Member Posts: 7,379
    Ditch the Nest. They don't play well with steam - even if the wiring is right.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mstrbill
    mstrbill Member Posts: 37
    There is only one ( 1 ) method that works for Nest shared with AC . It is - one power source from the AC side like you have , and add an isolation relay at the boiler to act as the boilers stat , and powered from the AC power at the Nest - have to splice the c - common behind and to the Heat white and Nest C , then red heat to W - Nest , this operates the relay at the boiler . Commission nest for gas heat and true age of house - no problems ever again . Nest is a fine product - never use two power source to it .
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
    Let us know if you have good luck with the Ne$t when it controls your boiler.
    The problem is that steam systems function better at a constant temperature, and the Ne$t can’t seem to learn that.—NBC
  • dbrown
    dbrown Member Posts: 8
    I personally used a Nest to control a boiler for over a year. I don't think you'll have any issues as far as control goes once you get the wiring figured out. The Nest figured out how the system responds and was always able to hit my desired temperature without overshooting.

    Mine worked great until the base failed and called for heat continuously for three days (I was away). Inside the house got to 113F, killed my fish and some houseplants. Called up Nest and they said that was a known issue with second generation and offered me 20% off a new one. Thanks Nest. Bought an Ecobee and I love it. It's way more nerd friendly than Nest.