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Heating, Air Conditioning, Fridge, HVAC/Wiring Nest Thermostat with dual-fuel heat pump/furnace system



I would like to replace my existing Honeywell TH9421C1004 VisionPROŽ IAQ thermostat (along with its accompanying Honeywell THM5421C Equipment Interface Module (EIM)) with a newly purchased Nest 2nd Generation Learning Thermostat.

I have a dual-fuel heat pump system with York components:
York LX Series YHJF36S Heat Pump
York LX Series TM9V080C Furnace (natural gas)

Right now, there are only the three wires between the EIM and the thermostat (labeled "1", "2" and "3" at the EIM, and labeled "R", "C" and "Data" at the thermostat).  I can back out this 3-conductor wire between the thermostat and the location of the EIM near the furnace, and replace with however many conductors I need between the furnace/heat pump and the Nest thermostat.

My question is around what I connect to the various terminals on the Nest thermostat.  Within the EIM right now, I have these terminals that have wires connected:

Terminals with only one label:
"R" - Red wires going to furnace and heat pump
("R", "Rh" and "Rc" terminals are jumpered together)
"C" - Blue wires going to furnace and heat pump
"Y" - Yellow wires going to furnace and heat pump
"G" - Green wire going to furnace only

Terminals with two labels (one for conventional heat source, one for heat pump):
"W1" (Conv.) / "O/B" (H/P) - Orange wires going to furnace and heat pump
"W2" (Conv.) / "AUX" (H/P) - White wires going to furnace and heat pump
"W3" (Conv.) / "AUX2" (H/P) - Brown wires going to furnace and heat pump

This terminal does not have anything connected:

I've been assuming I'd connect these wires from the EIM to the Next thermostat terminals:
- Red wires (connected to "R" in the EIM) to "Rh" on Nest
- Blue wires (connected to "C" in the EIM) to "C" on Nest
- Orange wires (connected to "W1"/"O/B" in the EIM) to "O/B" on Nest
- Yellow wires (connected to "Y" in the EIM) to "Y1" on Nest
- Green wire (connected to "G" in the EIM) to "G" on Nest

What I'm not sure about is where to connect these wires:
- White wires (connected to "W2"/"AUX" in the EIM)
- Brown wires (connected to "W3"/"AUX2" in the EIM)

The Nest Pro Install guide ( does provide some diagrams for connecting wires, but I'm not sure what is the most applicable diagram for my situation.

Thanks for any help you can provide!



I'm not firmiliar with Nest 2nd generation stats ,( there are literally hundreds of thermostat models that are produced, I am not firmiliar with all of them ) briefly looked Nest 2nd gen. up , says it's compatable for a dual fuel system. I'm not 100% sure but the W3 may go to the star terminal on the t-stat. The nest will ask you for what type of configuration you have once a wire is hooked to the star terminal. W2 may also go to the star terminal and the W3 is not used. I just don't know.

I do know

Red wire hooks to R at t-stat and R at furnace and R at outdoor unit.  That is one side of your 24 volt ac power from your transformer.

C common is the other side of the transformer looks like you designated the blue wire ,so C from stat goes to C at furnace and to C to the outdoor unit. I believe I read at Nest website that  99% of time C may not be needed. I still like to hook up C anyway.

Orange wire typically is for the reversing valve so that wire hooks to O/B at the stat and goes to the outdoor unit O hookup .

Yellow wire hooks to Y at the stat then goes to Y outdoor unit for the compressor contactor.

Green wire hooks to G at stat and needs only  hook to the furnace G for the blower fan. Does not need to hook to any connection at outside unit.

Your furnace has a 2 stage heat, I do not know how York two stage gas valves energize ,( I do not sell York )do they use one W wire to control both stages ?? or does your system have a W1 for low stage and W2 for high stage ??? ( W wire being for heat )one of those W wire goes to outdoor unit ( when outdoor unit goes into defrost, a contact closes on the defrost board and brings on the furnace to temper the air, once defrost is over the defrost board opens the contacts and shuts off the furnace.

I do not know how the Nest stat knows when the Outdoor Temperature goes below a preset balance point and switches from heat pump mode to Furnace. I did not see an optional OD sensor to be hooked up to the Nest. Your old stat had an OD sensor option.

Also when your in Heat Pump mode and when your outdoor unit goes into defrost does your present system bring on the furnace to help temper the air ? ( as you know your basically in A/C mode while in defrost and your supply temp. can get extremely low )

Does the Nest account for this ?? I do not know.  

Personally I would try calling Nest technical rep./assistance and ask them how to wire. You may have to play phone tag but any time I had to call Technical Services of any Company I did get through. Hopefully they will answer question from DIY people. Some Companies will only deal with Contractors. You can also ask another expert here.

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40+ years diversified experience.An HVAC/Refrigeration Self Employed Contractor since 1986. NATE Certifield. Answer questions pertaining to Residential and Commercial Air Conditioning , Warm Air Heating, Heat Pump systems.Mechanical and Electrical troubleshooting of these systems.Be as detailed as possible when describing problem.Packaged unit or split systems.No appliance questions.


Installation and Troubleshooting, Mechanical and Electrical, in following areas, Refrigeration ( Walk In and Reach In Coolers and Freezers ) Commercial Roof Top and Packaged Heating/Cooling ( Natural Gas,Propane, Electric, and Heat Pumps ) Computer Room Air Conditoning Systems,Commercial Residential Packaged and Split Systems Air Conditoning and Heat Pumps.Warm Air Oil , Natural and Propane Gas Heating systems.

Graduated from Private Technical School in 1975. An HVAC/Refrigeration Contractor . Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Master Heat Pump Technician 1986. Served 33 years combination Active Duty Air Force , Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop/Mechanical Shop. Troubleshooting/Installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Senior NCO ,and a Mechanical Superintendent.

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