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Which is my "C" wire, want to install a new Hoenywell RTH9580


I like the look of the new Honeywell RTH9580 for my oil, one-pipe steam. But it needs a C wire. I've got about 10 wires going from the thermostat to the burner control, but only 2 are in use currently with my 1970s dial thermostat. So I'm looking to figure out where I can find the C wire on the burner control. I've attached pics of the burner control, and I'm sorry that some are blurry, if you need me to read and post what some of the words are on the control let me know. Thanks in advance for any help.


  • JStar
    JStar Member Posts: 2,752

    Can't be done with only what you have. I suggest adding an R8845U relay that has a common terminal for t-tats.
  • billtwocase
    billtwocase Member Posts: 2,385
    "C' wire

    there is only a "R" and "W" on that control. Do you have a separate AC unit utilizing one thermostat?
  • C - wire

    Nope, No A/C. The thermostat is simply an on-off for the boiler. But the new thermostat that I want to buy has a screen, displays outside temp (via internet), and has wifi. So it needs a C wire.

    If you could recommend a specific control/relay that I need to buy, then I'd order it, and I'm sure I would be able to figure out the wiring. But I am afraid of buying the wrong thing from the get-go.

    Is what I need going to replace the Carlin box (that's a relay, right? Or a control?), or be in addition to it?

    I am really not experienced with controls, but I pretty sure with a little research and help from this board I will be able to install it.

  • JStar
    JStar Member Posts: 2,752

    The R8845U relay is a separate switching relay. Feed it with constant 120V, and tie in your t-stat and boiler wiring. Simple.
  • Paul48
    Paul48 Member Posts: 4,470

    SR501-4 might even be cheaper, and has the 24v common.
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