Just received a Nest thermostat I would like to install. My system is a McLean boiler steam to radiators. My existing setup has a 2 wire thermostat connected to boiler. From thermostat the R wire goes to boiler Y and the W wire from thermostat goes to G on boiler. I’ve added a C wire and tried numerous combinations on Nest thermostat. I’m receiving power but none of the different combinations I try on Nest seem to work. Do I need to change the Boiler wiring?

The picture I’m going to attach will be a bit confusing since the red/white wires were spliced to 2 black wires.

  • Here's the rub -- what are you trying to accomplish with putting the Nest on your system? A one-pipe steam system is a slow system, running best when allowed to cycle for long periods of time and maintain a steady temperature throughout the house. Fancy vacant-modes etc aren't very useful to you, and may leave the house decidedly uncomfortable (or worse) due to maloperation of the system (such as steam not reaching all the radiators) Feb 12, 2021 at 0:18
  • @ThreePhaseEel You can adjust the minimum cycle length. The thermostat can learn how long it takes to heat up to temp, and you can turn it on via the internet or turn it down when you decide not to come home.
    – gbronner
    Feb 12, 2021 at 0:53
  • @gbronner -- what I'm trying to say is that it may very well be more efficient to leave the system running at a set temp than to try to have it setback aggressively during night or unoccupied periods. Unless you're dealing with a sort of "vacation mode" scenario, the Nest isn't much help for systems that prefer to run at a constant temperature Feb 12, 2021 at 1:02
  • @ThreePhaseEel at least where I am, most of these systems have rudimentary timed thermostats. apart from short-cycling the boiler, and the long windup time, no reason not to. Also, at least here, the steam systems are massively overdesigned -- they were built to pump out enough heat to allow you keep the windows open and get fresh air. Also, see Radiator Labs for a different approach to this problem
    – gbronner
    Feb 12, 2021 at 1:06
  • 2
    Does this answer your question? Nest snow thermostat with a McLain PEG-45 boiler Feb 12, 2021 at 1:21

1 Answer 1


Unfortunately, on non-electronic boilers, labels are all over the place, and the thermostat is reversible -- it doesn't matter which wire coming out of the thermostat is connected to each of the two terminals on it.

The basic principle of how the furnace works with a dumb thermostat is:

24V Transformer (R)  --->  (wire 1) Thermostat (wire2)  -> Relay -> 24V Transformer (C)

It will also work with the following configuration:

24V Transformer (R)    -> Relay  --->  (wire 1) Thermostat (wire2)  ->24V Transformer (C)

So the first challenge is to identify the (R) and (C) terminals on the 24v side of the transformer.

From there, you need to trace the (R) wire and see if it goes to the transformer first or the relay first.

If it goes to to the thermostat first, that's your (Rh) wire, and the other one is your (W) wire (you can check this with a multimeter again -- at the thermostat one wire will have voltage and the other won't). Find the other end of the transformer and attach the (C) wire to it as well as to the Nest's (C) terminal.

If not, you now have your (W) and (C) wires -- find the (R) side of the transformer and attach your wire to it. At the thermostat, plug it into the (Rh) terminal.

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