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I have a standard gas furnace, but recently installed an electric radiant floor in one room. I have a transformer at the electric floor and a regular low voltage thermostat wire to my Nest thermostat.

I can wire the floor wire into the Nest as an 'auxilary' heat, and specify that it is electric radiant, and both heat sources work as expected. However, what I really want is for the floor to kick-in when not at a higher temperature than the gas furnace does, so the floor can be warm even if the gas furnace is not required to heat above its separate temperature.

I believe this is the entire purpose of a heat-pump (to turn on and heat at moderately low outside temperatures, but use additional heat sources when required). The Nest supports wiring for heat-pumps. What if I just wired the electric floor as a heat-pump and told the Nest that's what it was? Are the signaling voltages any different?

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    Is there a reason you don't have the radiant floor on its own 'stat in the room with the radiant floor? – ThreePhaseEel Jan 24 '17 at 1:29
  • @ThreePhaseEel nope, I could have done that. I guess my ideal is not having to think about it. – Sealander Jan 24 '17 at 1:35
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Just remembered that I never circled back here.

I was able to get this behavior by wiring the floor as "Stage 1" heating and the main furnace as "Stage 2". To do this, the white wire from the floor heat goes to "W1" and the white wire from the main furnace goes to "W2/Aux". My "common" floor (green in my case, but not well standardized) wire went to "C" (optional) and my green furnace fan wire went to "G".

And the red wires? This was tricky. Normally the Rh and Rc terminals are short-circuited but the Nest manual says that the Nest does this automatically. However, all combinations of the furnace and floor red wires to either/both of Rh and/or Rc didn't work until I manually connected the two to a single red wire and connected that to Rh.

Final wiring: enter image description here

Now, when the Nest needs to increase the temperature a few degrees it will only activate Stage 1 (the radiant floor), but when it needs to raise the temperature a lot (like in the morning or when it's very cold out) both Stage 1 (floor) and Stage 2 (furnace) activate.

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