I spoke with an ecobee tech and they pointed me to an alternative solution. If I get an adapter called a "Fast-Stat Common Maker", all I need to do is figure out where to connect the C-Wire at the furnace board and it should do the trick.

So question now is: Where can I tap into this wiring setup to get my 24V C-Wire feed?

Original question:

We'd like to install a smart thermostat in our home which is heated by a central Wood/Electric furnace (Wood being the primary heat source).

Currently, there is a group of 4 wires running from the furnace's command block to 2 thermostats on the main floor.

2 of the wires go to a simple mercury-tube thermostat which controls the dampers for the wood heat. (We keep this one at 26C so that it's always open if the fire is burning).

The other 2 wires had an identical setup (with a mercury thermostat) for the electric heat source, but a few years ago we replaced it with a programmable Honeywell thermostat so that we could program the electric heat when we were away for a few days (when we obviously couldn't make fires to heat the house). I've got this one wired very basically: 1 wire to 'R' and 1 wire to 'W'.

What we're hoping to setup now is a smart thermostat so we can see/control the electric heat from our phones when we're away. (We don't need the Alexa/Siri functionality)

Only problem is every smart thermostat I've found requires a "C-wire" which isn't present in the current configuration. The Ecobee3 Lite comes with a Power Extender Kit, but the instructions still reference control board wiring letters that aren't present at the furnace.

The wiring diagrams for my furnace can be found in the manual here: Page 18 or Page 23 (I'm just not sure what blower motor I have, but it's the PSG-4000 Wood/Electric Combo).

Is it even possible to hook up one of these thermostats?

Are there alternative thermostats that have the Wi-Fi functionality but don't require the C-Wire ?

Bonus question: For the thermostat that controls the wood furnace dampers, is it ill-advised to convert that to just a simple on/off switch?

EDIT: Here is the wiring diagram: Wiring Diagram

  • Can you post a wiring diagram for this hot monster of yours, or pictures of the wiring at both ends for that matter? Sep 23, 2018 at 21:45
  • @ThreePhaseEel: Added the diagram
    – JJT
    Sep 23, 2018 at 22:02
  • Is your system heat-only, or is there central air involved as well? Sep 24, 2018 at 0:14
  • It's heat-only.
    – JJT
    Sep 24, 2018 at 12:00
  • I take it both 24VAC transformers are pretty bog-standard, chassis-mounted units? Sep 24, 2018 at 22:56

1 Answer 1


You have two 24VAC transformers here, and that's where the fun starts

Your system is unusual in that it has two 24V control transformers within it -- one for the electric side that powers the relay and contactor, and a second for the wood side that powers the servo motor. Furthermore, the transformer for the servo motor is half the size of the transformer for the electric side of this furnace (20VA vs 40VA), limiting its ability to power a power-hungry WiFi thermostat as well. Given that the wood-side transformer has its C wire exposed at the control terminal block and the electric side transformer doesn't, that leaves you with two options:

  1. Replace the 20VA 24VAC transformer with a 40VA version (40VA HVAC control transformers are utterly bog standard) and then tap C at terminal 6 on the furnace's control terminal block, using a 24VAC SPST relay to allow the thermostat to control the electric side.

  2. Tap the electric-side C terminal, likely at terminal 7 on the relay, and then use a 24VAC SPST relay to allow the thermostat to control the wood side.

Something like this would suffice for this application as far as relays go (technically, it's not UL listed for standalone use, but for mounting into a furnace, that's not a big deal).

  • Thank you. I'll try the second option first and report back.
    – JJT
    Sep 25, 2018 at 1:30
  • Is the SPST relay required? Could I just tap into the terminal 7 directly, for instance?
    – JJT
    Sep 25, 2018 at 17:03
  • @JJT The relay is required to bridge between the two different transformers involved here Sep 25, 2018 at 22:20
  • I'm having a hard time understanding where the relay would be added to the circuit. I was under the impression that the second thermostat that I currently have to control the wood was on it's own circuit, independent of the electric elements circuit.
    – JJT
    Sep 26, 2018 at 11:28
  • @JJT -- it indeed is on its own circuit -- the relay lets one circuit control the other without making a direct electrical connection between the two circuits Sep 26, 2018 at 11:35

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