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We have

  • T874G1196 thermostat by york (can only find honeywell documentation)
  • MB7BM-1200K-B air handler
  • H8HK015H-21 electric heater
  • Thermopump (heating and cooling)

We want to connect it to a nest thermostat E.

We have 7 wires to connect to the nest. This is a picture of our current thermostat wiring. enter image description here

I spoke with a nest employee via chat on their website and she seemed to say that our system was compatible only we would need to get a pro to come and "join the O and B wires into the "*ob" terminal on the nest". She also mentioned something about possibly needing a relay but was reluctant to go into detail as I felt she was worried I might mess up my system.

Before we go and blow a whole wad of cash for some guy to come and twist 2 wires together, I just wanted to get your opinion on our setup.

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  • here is a description of the hvac wires ... highperformancehvac.com/thermostat-wiring-colors-code ........ it implies that O and B are the opposites in function, depending on the heat pump manufacturer ...... one type cools by default and requires a signal to switch to heating mode .... the other type heats by default and requires a signal to switch to cooling mode
    – jsotola
    Jan 3 '19 at 3:14
  • Can you post photos of the wiring at the air handler, as well as the wiring diagram for your air handler? Jan 3 '19 at 3:21
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Thermostat User's Guide, pg 168, Fig. 160

Schematic and hookup of T874G with Q674J

Here is the wiring for 1 stage heat with aux heat and emergency heat from the Nest E Pro Installation Guide

1 Stage Heat Pump with Aux Heat and Emergency Heat

I don't think you're compatible with the Nest E since you have separate controls for both auxiliary heat and emergency heat. A Nest Learning Thermostat would be compatible:

1 Stage Heat Pump with Aux heat and Emergency Heat

If you had a Nest Learning Thermostat, you would wire:

  • G -> G
  • W -> W2/AUX
  • R -> Rc
  • X -> * (Emergency Heat)
  • B -> C
  • O -> O/B
  • Y -> Y1
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  • Confirmed wiring with my Nest 3 and heat pump, Matt is correct that I think you need a Learning Thermostat and use * for emergency heat.
    – Haney
    Jul 22 '19 at 18:19
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here another case that also works

the configuration for Nest will ask you, what color is the Orange cable, in my case as you see here my cable is orange. in my case also the brown cable is not used, and the white is used for W. and what are you configuring for X-> *, the option Emergency Heat should show up.

New Nest Learning Thermostat

and the old Q674J 1027 Q674J 1027

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  • 1
    "what color is the Orange cable" Won't the orange cable always be orange?
    – JACK
    Jun 9 at 18:08
  • according to the info in the question, sometimes is blue instead of orange.
    – E. Rivera
    Jun 13 at 3:27

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