I have an old Honeywell Q674 thermostat with mercury in the house we just moved into.

I'm attempting to upgrade to the nest 3rd generation thermostat, however the wiring labels are VERY different. I believe I have figured out that:

  • W2=w1
  • G=G
  • O=OB
  • Y=Y1
  • E=aux
  • X=C
  • R=RH or RC

Does there need to be a jumper from rh to 3rd? Does W2 go to W1?

Amy I do would be helpful. Here are some photos.


Based on this wiring diagram, this looks like a multistage heat pump thermostat (provide control of a two-stage heating/one-stage cooling heat pump system).

From left to right in your picture:

  • W2 = Aux. Ht. Relay
  • G = Fan Relay
  • R = Power
  • X = (Note says terminal labeled C/X on some models, and based on diagram, I think your assumption of common wire is correct)
  • E = Em. Ht. Relay
  • Y = Compressor Contractor
  • O = Cool Changeover Valve

Based on those wires, and this wiring from the Nest Pro Installer Guide, I think you'll wire it as a 1 Stage Heat Pump with Aux heat and Emergency Heat

1 Stage Heat Pump with Aux heat and Emergency Heat wiring

So if we link up your current wiring to Nest:

  • W2 -> W2/Aux (Auxiliary Heat Relay)
  • G -> G (Fan Relay)
  • R -> Rc (24VAC power)
  • X -> C (24VAC Common Wire)
  • E -> * (Star: E)
  • Y -> Y1 (Compressor Relay (Stage 1))
  • O -> O/B (Heat Pump Changeover Valve)

I just went through this replacing an old stat with a new smart ecobee. It was a pain, but I figured it out with some help online. One tip, check the compatibility test online, there you tell them what wires you have and it will tell you if you're good, or what you need to do.

In my case, I had to actually connect a C wire to my furnace so I could get continuous power to the stat. What I learned was the stat wires are less important than what is actually attached on the board at the furnace.

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