C-Wire for a Vulcan Gas Central Heater for Nest Thermostat-E

Hi All,

I'm hoping the friendly people here could help me.
I'm looking to purchase a Nest Thermostat-E but my current thermostat (Vulcan/Bonaire Summer Breeze) only has 2 black wires connected.

I've searched around and asked for advice and I've heard some people say it will work with just 2, and some say I need a C-Wire.

Given the opportunity, I think it best to run a C-wire if possible, I have uploaded images of the Heater unit and it seems as though the 2 wires from the thermostat connect to the 2 x "Heat" terminals and I assume the 2 x "Cool" terminals are for Air con if I had it.

Now my question is, is the single 24v terminal used for the C wire? And if so, what would the 2x black wires be named when connecting to the Nest Thermostat?

The heat I have is a Vulcan Tubular Central Heater – V3 20i Internal NG : Part #5310052

Images are here: https://imgur.com/a/WveSexP

Thank you all!


  • Yes/ definitely Denzel and Kobe.

    -im an expert in almost everything.

    You're welcome

    • Thanks Scatman00
      I thought it would be the case, I read that post but because the labeling on mine is different I wanted some confirmation on which cables they'd be and if that 24v connector was definitely the C-wire.

      • The most important thing is naming the black wires.

        • Unfortunately I don't know what they would be as the output from my heater just says Heat.

    • I can only talk from my own experience because I don't have the same system. A year ago I installed a Nest Thermostat on my Brivis central gas heater to replace an old Honeywell manual mercury controller. It only had 2 wires, with no C wire. It works fine for me. The battery hasn't gone flat, even though I haven't used the heater since last winter. It doesn't look like the thermostat is activating the heater just to keep its battery charged.

  • yes, you'll need a c wire, and also a another wire called rc to hook up to the nest because C wire alone will not be sufficient to charge up your thermostat when your system is running. That was my experience with the higher model nest learning thermostat but I suspect the E one requires the same thing. Easiest way is to find 24vac adapter from jaycar and use the ground wire as your c and the other as rc wire. Or if you can open up your heater/cooler and have a look inside the controller board you might find both of those 24vac output terminal somewhere near your 2 existing black wire, something like this from reddit

    Initially I only hooked up the c wire but once your system is up and running it will actually connecting 2 black wire together to complete the circuit, doing so it will reduce the voltage difference between the c wire and any of the other 2 wires to something like 18vac, which will not be enough to charge your thermostat. Without an active wire linked to Rc, your thermostat will be consistently in low voltage state and it will limit some of its function like the automatic display when someone go pass (for the learning thermostat, not sure if your E one has this functionality).

    • Thanks for the insight lgacb08,

      I'm still a bit confused, could you please have a look at the images and advise, I've just uploaded what should be the circuit board diagram and it looks as though there should be a free active and neutral point towards the bottom right.

      Could you please answer a few questions if you know the answers.

      Does that mean I need 4 wires in total?
      there are 2 coming from the green wire crimp points, any idea what they should be labelled as?

      There is a point labeled 24v, is this what is commonly called the C wire?

      Looking closely, I just noticed there is a small metal ring/pin going from port 2 (heat 2/2) to the 24v port, what is this doing and do I need to remove it if I connect a wire to this port to use as the C-wire?

      So if I connect it to the board, then active would b RC and neutral will be C?

      Thank you very much for you help!

      • well, forget what I wrote then, your board is different to mine (braemar). So you might get away with 3 wire. The green cimp points you mention, based on the diagram show 2 of your heating wire connecting to the left, the middle one supply constant 24vac so that one might correlate to Rc, but because it got a pin connecting to port 2 meaning one of your black wire also has constant 24vac so the Rc wire is redundant. You should probably leave the pin as it is.

        2 of your existing wires should go to to rh and w (w1).

        The issue is common wire because it's not on your thermostat port, you could get away with it by get another wire connecting to the metal casing of your system, or if you want extra safe get it from the neutral wire of a proper 24v transformer. To be extra safe, you should get a multimeter and observe changes in voltage between your wire to determine what to connect. And test out the voltage between one of your existing wire with the outer casing of your heater (or another neutral wire from a proper transformer), if you can ensure there's at least 18vac between any of those (when your system is either on or idle) then your nest should be working properly. The reason for C wire is because when those 2 wire thermostat activate the heater, all it does is connecting these 2 wire to complete the circuit, so if you use 2 wire for the nest, voltage difference between the 2 wire will become 0 and it is not sufficient to power/charge the battery. Nest is smart enough to check which wire it should use to power the system so it actually doesn't matter which of your existing wire should be rh or w.

  • Hi Raiyani did you go for the Nest in the end, I have the same combo as you, but also found a sticker under the terminal blocks that shows the 1terminal = R & 2 terminal = W, also terminal 4 = G, numbered left to right as shown on your forth photo

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