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Nest to steam

I have a steam boiler with a 8184G Relay and would like to wire a nest thermostat  to it but need help on what to wire the third wire to.


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,177
    edited November 2021
    One of the F terminals is common I will look it up

    T T and F F are not sufficient for a thermostat that requires a common. I'm not sure the 24V transformer in this control is strong enough to power your thermostat. Bur it might be for here is the diagram.

    T closest to the edge of the control is R
    the other T is W

    F closest to the other edge of the control is C
    the other F is the flame sensor circuit.

    If the control fails I would recommend that you upgrade the control to R8184M as a minimum upgrade because the transformer in that control is more powerful. But others on this forum will strongly recommend a "Smart Control" like the Carlin 70200 with a Kool Kit option. This will give you extra diagnostic features for your oil burner. Honeywell and beckett also have Smart Primary controls to replace the R8184G. I just liked the Carlin the best for my customers on service agreements. Less failures in my experience. Longer warranty.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Rogerd
    Rogerd Member Posts: 4
    Ok thanks 
  • Rogerd
    Rogerd Member Posts: 4
    What is the kool kit option
  • Rogerd
    Rogerd Member Posts: 4
    Will using the F terminal affect the safety lock out?
  • Steamhead
    Steamhead Member Posts: 16,283
    That R8184G control no longer meets Code in many places, because its lockout time is too long- 45 seconds.

    The newer controls- Honeywell R7284, Beckett GeniSys, Carlin 70200- lock out after 15 seconds if the flame doesn't catch.

    The other problem with wiring the C wire to the primary on a steamer is that the limit controls (pressuretrol, low-water cutoff) typically cut power to the burner when they trip. This can cause this type of thermostat to blank out and possibly lose its settings.

    I like to wire a fan-center relay such as the Resideo (formerly Honeywell) R8285A1048 to the main power feeding the boiler, before any limit controls. Wire the R wire from the thermostat to terminal R on the R8285, the white wire from W on the thermostat to G on the relay, and the green or blue wire to C on both units. Then take the normally open wires from the relay and wire these to TT on the primary. This will make sure the thermostat always has power as long as the power to the boiler is on.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    EBEBRATT-Ed Member Posts: 13,851
    The transformer in your primary control is not nearly strong enough to power a Nest thermostat reliably.

    You either need to add a seperate transformer and relay or a fan center as @Steamhead suggested