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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.

  • 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 at 3:14
  • Can you post photos of the wiring at the air handler, as well as the wiring diagram for your air handler? – ThreePhaseEel Jan 3 at 3:21
1

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
  • 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 at 18:19

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