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Need a C connection at my Nest but boiler controls have no common

theONEendONLY Member Posts: 51
edited February 2021 in Thermostats and Controls
I just moved into my house and there is an osb-3 steam boiler in the basement. The controls are all 120v except for two wires meant for the thermostat (R and W)

After installing the nest thermostat, I noticed my boiler system would "pulse on/off" very quickly once every maybe 30-40 mins

After researching the issue I've come to the conclusion that the thermostat is charging itself off the connections to the boiler and making it act weird like that. Normally it would charge off the common wire which would constantly give the thermostat 24v (right?)

So my question is.... Where can I get 24v to bring to the C terminal in my thermostat? To me... It seems like I have to install my own transformer and run a wire to the C terminal. Can I just tap off one of the 120's from the boiler and use it to run my step down (to 24) transformer for the C connection? Would that take away too much voltage?

Any other ideas? Am I missing something in the wiring diagram that shows where I can send a common wire to the tstat?

Here is the link to the boiler. The installation guide has the wiring diagram and my boiler is fitted with the genisys 7505 burner


Really appreciate the advice.


  • Jamie Hall
    Jamie Hall Member Posts: 21,093
    You're not missing anything. There are a number of auxiliary power supplies on the market -- including, I believe, one by Nest itself.

    That said, you will find that the Nest is quite possibly the worst choice for controlling a steam heating system you could possibly make. They can be made to do a so-so job of it, but you must disable all of its learning features and set it for -- I believe -- true radiant. It was never designed for steam -- or, or that matter, hot water or radiant -- and never intended for it.

    Good luck.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 3,864
    The only thing good about the N E S T is the Marketing that makes consumers "Think" there good.

    News Break there not!
  • theONEendONLY
    theONEendONLY Member Posts: 51
    Thanks for the replies, but can you tell me what makes them so bad?

    The sellers had a basic $20 thermostat installed so I think nest would be a big improvement?
  • Zman
    Zman Member Posts: 7,446
    If you must use the Nest and you are able to get a 3rd wire to the t-stat, I would use one of these https://www.supplyhouse.com/Taco-SR501-4-1-Zone-Switching-Relay
    You just power it 120 volts, wire the Nest to R,W,C on the top right and the existing wires from the boiler to terminals 5 and 6 on the bottom right. Its a transformer and a relay all in one package.
    You don't want to add just a transformer into the mix or you may let the smoke out of the burner controller's internal transformer.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 13,424

    If you know a little electrical you can use your Nest. My nephew put a nest in for his steam boiler and struggled with it for a year with no C wire. It's an easy fix. You need a 120/24 volt 40va transformer and a Rib #RIBu1C relay. You can get both of those at Supply House.com

    Mount the transformer somewhere where you can get 120volt power easily. You can get it from the boiler circuit or any other circuit. So hook 120 v to the transformer Black-Black, white-white.

    Mount the rib relay on the same junction box you mount the transformer on.

    You need 1 3 wire cable run from this junction box location to the Nest.

    Run a 2 wire cable to your boiler from the same location.

    1. On the common side of the transformer connect the C wire from the Nest & the white/yellow common wire from the Rib relay.

    2. On the others side of the transformer run a wire to RH on the Nest (use the red wire)

    3. Run the W (white wire) wire from the Nest to the Rib relay and connect to the white/blue wire

    4. Now you just have the 2 wire cable left.
    connect 1 wire to the Rib Relay common contact and 1 wire to the Rib relay Normally open contact.
    So 1 wire goes to yellow on the Rib and the other wire goes to orange on the Rib.

    Over at the boiler disconnect your old T stat wire and connect this new wire. It doesnt matter which terminal get which wire ....doesn't matter.

    Then your done

  • Zman
    Zman Member Posts: 7,446
    ^^^^ Same solution, less expensive, more work....^^^^^
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 21,093
    @theONEendONLY -- you enquire as to what makes the Nest so bad. Like so many things, for what it's designed for it's not bad at all. Overpriced yes, but it's actually very good. For forced air heat. Forced air heat responds to thermostat changes very quickly, or at least feels as though it does, and the Nest is designed and intended to vary the temperature in the space in response to its use. It does this by sensing room occupancy and also the occupants desires.

    That's wonderful. Problem is, most other heating systems don't respond that fast (the extreme being radiant, which may take days -- but ;even an oversized steam system can take a couple of hours to change the temperature by 5 degrees) and the poor Nest keeps trying to raise and lower the temperature as people come and go -- and both it (trying to "learn" behaviour) and the heating system trying to hit a constantly moving target get confused, for lack of a better word. So they wind up fighting each other -- and using far more energy than they should.

    The Nest does have settings to disable all the learning and occupancy figures, and to limit or eliminate setbacks, and to understand, if you will, the much slower response characteristics of anything but forced air. They don't come programmed that way, though, so the homeowner or installer has to do it -- and usually doesn't, or can't figure out how (the instructions are, to put it charitably, opaque).

    I personally have other problems with it, not particularly wanting the Google and the rest of the internet to know my usage patterns and occupancy, but that's a different issue.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England