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steam boiler wiring

dbrez8dbrez8 Member Posts: 5
edited July 2017 in Gas Heating
Edit: I should have put this in Controls. I cant figure out how to delete it. Sorry..
Discussion here:

I have a weil mclain steam boiler. PEG-30A-S-PI, nat gas, single pipe/zone. Trying to wire up a wifi tstat with a C wire.

Old tstat had w from transformer wired to w and y from transformer wired to R/Rc per this schematic.

Just ran 18/5 wire and trying to figure out how to wire up new t stat to the transformer properly with a C wire. Should I also connect up R from the transformer? Any help is much appreciated!


  • ratioratio Member Posts: 2,484
    The common on the transformer is marked "C" on the schematic you linked. I don't know that control well enough to guess whether the "W" or the "Y" is actually the 24VAC power. A battery–powered stat would work fine with them reversed, but a control–powered stat won't. I'd wire the W and Y to the same terminals on the new stat, add the C, & see of it works. If you have a voltmeter you could check which one (W or Y) has about 24VAC to the C terminal, that one would be the power, lands on the R/Rc terminal.
  • dbrez8dbrez8 Member Posts: 5
    Thanks. That was my plan. The W actually has 24 (actually 26) vac with reference to C. which confused me since the Y was plugged into the R/Rc previously, and the R is usually constantly 24 with reference to C. No?
  • HVACNUTHVACNUT Member Posts: 3,411
    I left a response in "controls"
  • SteamheadSteamhead Member Posts: 13,863
    HVACNUT said:

    I left a response in "controls"

    Me too.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
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