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Problems Connecting Boiler 3 Wire to a Nest Smart Thermostat

dpatz Member Posts: 1
I'm trying to update my thermostats around the house. They currently are old mercury-powered White-Rodgers, and I'm just looking to step up to a Nest Thermostat.

The problem I am having is that the new Nest I put on doesn't turn the boiler off.

I have three White-Rodgers zone valves, and from what I can tell, they need a longer signal to start closing than just the thermostat stopping the heat call.

The old thermostat ran off of three wires: red, white, and green.

Any solutions?


  • Zman
    Zman Member Posts: 7,553
    edited December 2018
    You should be able to make this work with 3 wires. Essentially, any t-stat is a switch. It just closes the circuit from red to white and tells the boiler to do it's thing. To make the nest work, it also needs to pull some power from red and send it back on a common. Your third wire needs to be wired to the common on the transformer. Pictures and a sketch of the wiring would help. There are dozens of ways to wire one of these. We need to know what you have.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ratio
    ratio Member Posts: 3,605
    It kinda sounds like the zone valves are three wire, power open power closed. IDK if the Nest will work with that style of zone valve.

    It could be made to work with a relay, but there are other thermostats that will work with 3 wire zone valves that are more favored here than Nest.

  • SuperTech
    SuperTech Member Posts: 2,127
    I would recommend considering a different thermostat. Nest thermostats are designed for use with forced air systems. To apply them to a boiler would be a downgrade over the white Rodgers. Do a search on here about it, you will see numerous other reasons besides the wiring that show that the Nest is a poor choice for a hydronic heating system.