I know this question has generally been asked but every system is different and I’ve seen nothing like mine.

I have a tankless water boiler for baseboard heat with a separate electrical control panel for a three zone system.

enter image description here

I want to add a C wire, hoping I can just plug it into one of these slots. But can’t make heads or tails of it. There is a point marked C up top. There are other markings for CP ZC bottom right. The wires leading out of the top are the zone thermostats on two-wire systems. (Two zones are two-wire heat only, a third is heat plus AC and fan.)

  • Do you have a make and model for your zone controller? Commented Nov 12, 2022 at 5:18

2 Answers 2


I think it says 'C' above the second terminal here:

enter image description here

Also judging from the tracks on the circuit board that's a good candidate for being "C"

also check R and W are the right way round, if you have them wrong your smart thermostat will not work

  • Wondered about that. But if I want to add a C wire to all three thermostats, can I use this one point and plug three wires into it? Commented Nov 12, 2022 at 0:19
  • there may be more C terminals for the other zones, follow that silver looking track and see where else it goes to. if you only have one C terminal you should join the three C wires into a single wire an put that single wire onto the terminal.
    – Jasen
    Commented Nov 12, 2022 at 0:26

You have two voltage here. The 120 Volt and the 24 Volt.

The key is to find out which is which, so not to use the white (neutral) from the 120 Volt.

The black cable with two thin wires in it (Red and White) is your 24 volt going to/from thermostat.

You can use the thin white wire nut for your C.

  • 1
    Sorry having trouble following you. Could you annotate the image or describe the rough location by any chance? I see a black wire with red and white coming out of it on the far right middle. There? And what do you mean by the thin white wire nut, not seeing that? Commented Nov 11, 2022 at 21:43

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