I’m looking to replace my existing thermostats (two zones) with smart thermostats that require a C-wire. The current thermostats do not have the C-wire connected, but the wire is already present in the wire bundle. I have both an HVAC system and an oil burner (Peerless WBV -03 Boiler). The oil burner has a wiring relay panel, where I can see the wiring for each zone, as well as the unused C-wire for each zone (green wire wrapped around the wire bundle). However, on the relay panel, each zone only has terminals for R and W wire connections (no C terminal). On the other side of the relay panel, there are a bunch of 120V terminals, some of them labeled as zones 1 and 2 circulators. The only two terminals on that side that are not cabled to are labeled ZC and ZR. I’m trying to figure out where I need to connect the C-wires from thermostat zones 1 and 2 to. Any help you can provide would be greatly appreciated. Please see the pictures of the oil burner relay panel I have uploaded.

Oil Burner Relay Panel Oil Burner Relay Panel2 Oil Burner Relay Panel3

1 Answer 1


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C is the 24V common, so it's probably at the terminal marked "COM" here.

  • Hi Jason, thank you very much for your feedback. I have one additional question that I'm hoping you can answer... When connecting the C-wire to the oil burner relay panel, do both the HVAC and oil burner need to be powered off at the circuit breaker panel? When I power off the oil burner, does that also cut power to the two thermostats?
    – Jim
    Commented Jan 23 at 13:40
  • Thermostat power comes though the yellow and red wires in the middle picture, so if you turn off the AC supply to this box the thermostats will be powered off.
    – Jasen
    Commented Jan 23 at 19:18

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