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QUESTION: Hello,

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:
"Y2"

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 (http://support-assets.nest.com/images/pro-faq/Nest-Pro-Installer-Guide.pdf?utm_s) 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!

Regards,
Mike

ANSWER: Hi Mike,

You will have to pull a new stat wire. To play it safe I would pull an 8 conductor. You won't be able to use the EIM.

Lets make this as simple a possible.

Terminal block in your furnace-----------Nest Stat

Red wire to R---------------------------R
Blue wire to C          C
Yellow wire to Y          Y1
Orange wire to O          OB ( There should be a switch on the stat, set to O.)
White wire to W2          W1
Green wire to G          G




The terminal in your furnace does not have "RC". The diagram I am looking at, the nest stat is showing a wire landed on "RC". I would jumper "R" to "RC" in the stat.

As for the brown and white wires, I would pick one of the wires left over at the stat and land on W2. Then wire nut new wire to the white wire on W2. The nest stat only has two stages heat and the EIM had three. I would trace the brown wire and see what it is wired to. It would be unusual to have three stages of heat.

I hope this helps. Let me know if you have any questions.






---------- FOLLOW-UP ----------

QUESTION: Jim, thanks so much for your help!  I really appreciate it.

As you suggested, I traced the Orange, White and Brown wires from the EIM to the furnace and heat pump.

The Orange wire from the EIM (connected to the "W1" (Conv.) / "O/B" (H/P) terminal) goes to the "O" terminal in both the furnace and heat pump.  This one seems straightforward.

The White wire from the EIM (connected to the "W2" (Conv.) / "AUX" (H/P) terminal) goes to the "W1" terminal in the furnace and the "W" terminal in the heat pump.

The Brown wire from the EIM (connected to the "W3" (Conv.) / "AUX2" (H/P) terminal) goes to the "W2" terminal in the furnace and the "W1/66" terminal in the heat pump.

Does this "W1/66" designation on the heat pump trigger any other thoughts?

Thanks again!

Mike

Answer
Hey Mike,

When I saw the "W1/66 I thought it was a typo on the diagram. I have never seen this designation before. Let's just call it "W1".

Can you check the heat pump terminal block and see if it has "W2 out", "W1 out" and "Y2 out", if so what color wires?

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Jim S.

Expertise

I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.

Experience

I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

Education/Credentials
I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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