I have an older furnace with only 2 wires at the thermostat "white and red". No issues at all with my system works as should. Thermostat is new Honeywell basic model not smart. I want to be able to turn it on fan mode only. I do not have an ac unit attached. I only want this option to help circulate air in my house. Cooler in the summer and in the winter we run a gas fireplace that roast us out of the living room but everywhere else is cooler.i have an electrical background but Zero funace knowledge. I know ill have to run a new wire 3 or 5 doesn't really matter have easy access. My question is how i would hook this up ive seen plenty of post "c" wire G wire and what I'm looking at doesn't look that straight forward or im over thinking this.enter image description hereenter image description hereenter image description hereenter image description here

  • Is pulling new thermostat cable an option? – ThreePhaseEel Sep 12 '20 at 20:44
  • Ok I was able to figure out how to add pictures .. the setting on my phone were messing me up .. but yes I can easily pull a new wire I know ill have to do this. I don't know what to do when I reach the furnace – Eric D Sep 13 '20 at 1:09
  • @EricD -- do you have a multimeter with a resistance function on it? – ThreePhaseEel Sep 13 '20 at 1:48
  • I could get one yes – Eric D Sep 13 '20 at 2:41
  • I usually add a switch at the furnace to pull in the fan relay. By doing this if it is cold enough the gas valve will still turn on when the temp drops below the set point, most people turning on the fan leave it on for extended periods so opening a door to the furnace closet or going out to the furnace is not a problem. – Ed Beal Sep 13 '20 at 15:52

Your furnace has fan terminals, thankfully

The good news is that your furnace has a fan (G) terminal and corresponding Rc and C terminals, found on the relay module depicted in your second photo. The bad news is that the combustion controller is totally separate from the fan relay in your setup, so you'll need to wire this a bit differently from a standard furnace. First off, your thermostat will need to be set up for two-transformer operation by moving a slider, removing a jumper link, or flipping a switch; the instructions should document how this is done. Once that is done, you can wire yellow to Rc, green to G, and blue to C at each end, with the white wire going to W/W1 on the thermostat and one T terminal on the combustion controller, and the red wire going to Rh on the thermostat and the other T terminal on the combustion controller.

Note that while a 5-wire cable will get the job done here, it may be a better choice to put a fatter thermostat cable, such as an 8-wire or even a 10-wire, in now so that the wires are available for a future HVAC upgrade, such as fitting a heat pump. Also note that if you're having trouble getting the heat to work, you may need to swap the red and white wires at the combustion control end (one of them is Rh and the other W, but your combustion controller doesn't differentiate between the two, while your thermostat just might).

  • So I get what you're saying somewhat .... the 2 wire I have (red and white) seen in the top picture stay where they are and the the others would go to the relay. Or they would all go to the relay .. or do I need a a pig tail back to where they were because I see a W and R on the relay also ? ... reading this again I think I can just add everything to the relay but want to make sure – Eric D Sep 27 '20 at 18:35
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    @EricD -- you should just be able to wire the green and blue wires to the relay then – ThreePhaseEel Sep 27 '20 at 20:00
  • Ok so to be clear new red and white to back where they weren and then green and blue go to the relay ? – Eric D Sep 28 '20 at 8:04
  • @EricD -- actually, you'll need an extra wire for Rc -- the R on the transformer needs to be wired to that, and then the red wire from the existing burner control goes to Rh, as per the wiring diagram -- I'll edit the post when I get time – ThreePhaseEel Sep 28 '20 at 11:47
  • @EricD -- updated my answer – ThreePhaseEel Sep 29 '20 at 0:09

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