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I want to hook up my Honeywell rth9580wf to a natural gas garage heater that only has R, W, and G (not in use). It's currently hooked up to a standard battery powered Honeywell thermostat. I've read up on other posts for installing furnaces without C terminals, and gave it a try without luck. Here's what I did: -Using a voltmeter I tested across the R terminal, and the red wire from the control board to the transformer. Each measures ~27V. -No matter the heater's running state, I measure a constant ~27V at the R terminal and on the post of the transformer. -Hooked up the G wire (which wasn't in use) to the R, and put this in the C on the thermostat. This did not power up. -Was about to run a wire off the post on the transformer to the C, but held up before asking for advice.

I'd like some guidance I'd anyone can assist? Thank you. enter image description hereenter image description here

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  • How did you make the connection between the C wire from the transformer, the C wire heading off to the t-stat, and the C terminal on the heater's control board?
    – ThreePhaseEel
    Commented Dec 2, 2018 at 21:23
  • There isn't a C on the control board. All I tried was running a second tstat wire from the R down to the C on tstat.
    – Adam Esquibel
    Commented Dec 4, 2018 at 4:15

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You'll need to make a connection at that quick-connect with the red wire going to it

You see that quick-connect terminal you measured 27VAC from R to? That right there is where the control board connects to C, and can be used as a tap point for a C wire with the correct hardware. In particular, you'll need a 6.3mm (0.25") stacked or piggyback quick-connect terminal (such as one of these) that accepts 18AWG wire and appropriate crimping and stripping tools.

To assemble this:

  1. Turn the power to the heater off.
  2. Strip the end of the wire (blue is typical, but color doesn't matter) that you want to use for your C wire
  3. Crimp the piggyback terminal onto the end of the C wire
  4. Remove the red wire you measured 27V from R to from its terminal on the control board. This is the C wire from the transformer.
  5. Install the C wire piggyback terminal's female part onto the control board C terminal
  6. Install the red C wire from the transformer onto the piggyback terminal's male part.
  7. Turn the heater back on, and enjoy your new smart thermostat!
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  • Thank you very much. So to confirm, am I going to the quick connect that is right next to the screw down R terminal (first picture) or the red on the transformer (second picture)?
    – Adam Esquibel
    Commented Dec 7, 2018 at 22:58
  • @AdamEsquibel -- either one would work, although it'd probably be easier to use the one in the first picture from a wiring-neatness standpoint.
    – ThreePhaseEel
    Commented Dec 8, 2018 at 4:51
  • Ok, so I tried hooking a secondary wire as suggested onto the quick connect terminal and ran it to the C on the thermostat. I also hooked up the R and W wires. The thermostat did not power up. I tested voltage to confirm, and I'm reading about 27V off my new wire. I tried feeding off the transformer at the 24V terminal as well. No joy. Not sure what I could be doing wrong?
    – Adam Esquibel
    Commented Dec 17, 2018 at 20:54

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