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I am going to help out a friend hook up their nest thermostat and as you might have guessed it's only a 2 wire at the location of the thermostat. Since the cheaper and newer version of the Nest thermostat runs off the neutral wire and only uses batteries as backup I need to run a new thermostat cable to the boiler. The Boiler is a Burnham mst396sl-hb and I have included pictures of the wire location at the boiler and my plan. I think its simple and looks straight forward but I figure I would just run it by all of you if you don't mind. I plan to buy 18/4 or 18/5 and just use what I need since I want the color blue for neutral and the 18/3 has green white and red only. Thank you all for your time and I did do a couple searches but they all seemed more complex setups than I have.

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    What benefit do you see your friend's boiler gaining from the Nest's features (vs. a standard thermostat)? Also, is your friend's boiler feeding a steam system, or a hot water (hydronic) system?
    – ThreePhaseEel
    Commented Oct 17, 2021 at 17:26
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    The benefit is that the friend can turn on the heat before coming home or turn it down if she forgets to before leaving.. In addition she can make sure that the boiler is actually running and I think an alert can be sent if the temp drops to low indicating a problem with the boiler. The boiler is steam.
    – Jason Rose
    Commented Oct 17, 2021 at 17:32
  • Is their system a one-pipe or a two-pipe steam system...?
    – ThreePhaseEel
    Commented Oct 17, 2021 at 17:42
  • I am not there to check but I know there is only one thermostat for the whole house if that helps answer the question. the house is under 1000 square ft so I "assume" its a simple setup.
    – Jason Rose
    Commented Oct 17, 2021 at 17:52
  • It's not a neutral wire. It's a "C" wire. This is 24 volt low voltage wiring, so we certainly hope 120V power isn't anywhere around!
    – Harper - Reinstate Monica
    Commented Oct 17, 2021 at 19:09

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Wiring-wise: you have the correct idea

Your proposed wiring scheme (R to R/Rc, G to W1, and C to C) is correct for the boiler you're working with; this can be deduced from the wiring diagram supplied on page 34 of the installation manual.

But, you need to be careful with the configuration

However, when using a Nest or other "smart" thermostat on a steam system, you need to be careful with how you configure the thermostat to avoid issues with comfort and undesirable excess fuel usage. In particular, the Nest doesn't have a standard deadband/cycles-per-hour setting; instead, you need to configure it for a "Radiator" system and enable "True Radiant" mode to get it to treat your system correctly.

This is because steam systems (and radiant floor heat, and some older hot water systems that use high-mass terminals) are much differently behaved from the forced-air systems most folks are used to. In particular, radiant and steam systems heat the large thermal mass of the room first, then heat the air, giving them an "inertial" or "flywheel" characteristic that makes them far slower to respond to a setpoint change than a forced-air system.

This also means that daily setback programming is often considered a poor option on these systems; they respond far better to "set it and forget it" setups, with setback only used for extended vacations and such. For a Nest, this may require turning off automatic home/away detection to prevent it from causing daily recovery swings, as well as configuring the thermostat as described above to avoid uncomfortable "short cycling" of the boiler.

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