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C wire on Williamson-Thermoflo boiler

Hi I have a Williamson-Thermoflo GWA series 2 gas water boiler.
https://www.williamson-thermoflo.com/sites/default/files/field-file/GWA Boiler Manual 550142784_1212_0.pdf
Thermostat is installed with 2 wires only. I have a Nest thermostat that sometimes has issues and it was recommended to have a C wire for better power supply.
Is it possible to get a C wire from this boiler? This is a screenshot from the manual, and it has a wire labeled C. Is this the correct wire?



  • Jamie Hall
    Jamie Hall Member Posts: 23,295
    If you work on it long enough, and if you are very good with a multimeter, yes. You could figure out which terminal on the transformer connector was common and which hot and hook the C wire to the common.

    Do it wrong and you will let all the smoke out of both the controller board and the Nest.

    A much better solution is to power the Nest off its own 24VAC transformer and use the thermostat output from it to control an SPST relay, then wire the NO contacts of the relay as if they were a thermostat.

    I feel I should warn you that Nest are only marginally compatible with hot water heat...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,841
    Yes, that's a common. 
    Can you get information off the transformer? It should be 40 va. If not then it shouldn't be used.