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Powering Nest thermostat using boilers’s transformer

ranzerox Member Posts: 48
Can I add my Nest thermostat onto my boiler’s transformer for power? Thanks 


  • Jamie Hall
    Jamie Hall Member Posts: 19,960
    Sometimes. It depends on the configuration of the boiler transformer.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 3,336
    ranzerox said:
    Can I add my Nest thermostat onto my boiler’s transformer for power? Thanks 
    Why a NEST?

    Any other t-stat is better!
    EBEBRATT-Ed Member Posts: 12,658
    Depends on the size of the transformer and what else the transformer powers
  • HomerJSmith
    HomerJSmith Member Posts: 1,823
    More information on your sys, please. Yes, of course you can, the problem is whether it would be successful.
  • parity_check21
    parity_check21 Member Posts: 4
    Ignore the people hating on the Nest, you can certainly make it work. I had an unused third wire in the thermostat line that I spliced into the common terminal on the 24v transformer relay. This wire should connect to the "c" on the Nest. The common wire is not energized, it is just a neutral wire to complete the circuit. The current comes from one of the hot wires, maybe the R wire. I had 27 volts, was able to power nest without issue. Disclaimer: I am not an HVAC expert, make sure to cross check any advice with a pro!
  • pecmsg
    pecmsg Member Posts: 3,336
    NEST even with a C wire does some crazy things!

    just search NEST issues. 
  • Big Ed_4
    Big Ed_4 Member Posts: 1,970
    "Slowly I turn, step by step "
    I have enough experience to know , that I dont know it all
    Alan (California Radiant) ForbesPeter_26
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,039
    The history of the Nest used with steam heating has not been good. It, like just about every other "smart" thermostat, historically has not been able to properly operate steam heating systems and most converted gravity hot water systems. Typical results of the "smart" thermostats include: about a 10 to 15% increase in fuel usage, uneven heating, and excessive wear and tear on equipment, piping and radiators causing early failure. The new ones maybe are better, but I really don't know.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • SteamBoiler
    SteamBoiler Member Posts: 34
    edited November 17
    I have a Weil McLain steam boiler that I am successfully controlling with a Nest Learning thermostat. I would strongly advise against using the boiler power alone. Boiler was originally controlled by a Honeywell mercury 2 wire thermostat connected to the boiler's GY terminals, downstream of the Low Water Cut Off. I did luckily have a spare wire in the wall that I was able to connect the Nest C to the boiler C, but I was getting too many 3 minute power-related short cycles that I ended up using an external 24V 40VA transformer with an isolation relay. It was maddening while I was debugging but it looks solid now. In general each heat call is about 20 minutes, and I have the Nest set to 65F overnight and recovering to 69F by 6.30am in one hour steps. With the old thermostat, it looked like it was short cycling the boiler and making too many heating calls - the basement and utility room were always hot and the house wasn't. Knowing what I know now, I would probably use an Ecobee since it has control of the dead zone - you can choose the delta offset at which the thermostat will call for heat below program point, and you can choose the minimum run time so you get good value for the heat call.

    Edit: added schematic

  • AdmiralYoda
    AdmiralYoda Member Posts: 496
    I'm using an Ecobee. I've found that it works pretty good with my 40 year old Peerless steamer. Runs about once an hour or so when its cold enough out. I like it because I have sensors all over the house and it takes an average house temperature to call for heat, not just the temp where the thermostat is.

    I don't use "Follow Me" or Geo-Caching, or whatever it is to find out when I'm on my way home. I just set it at 68 and leave it.

    Here is how I wired mine. I don't know exactly how the Nest is wired but I imagine it is similar. I've had zero issues.

  • KarlW
    KarlW Member Posts: 6
    I used a nest from 2012 to 2016 or so. It worked fine on my 2-pipe steam boiler system, but I ultimately switched away because it didn't have Homekit capability.

    I next went with an Emerson Sensi, a far more simple but Homekit controllable thermostat. It worked quite well.

    I've since gone nuts and imported a Netatmo thermostat and radiator valves. Although my zoning causes problems, as a system it works great. Based on therms used and heating degree days over a two-year comparison, I estimate I have 8% fuel savings

    I suspect the short answer from me is that if an off-the-shelf generic thermostat works, so will smart ones.