Wiring diagram for heat pump with 2-stage LP furnace as a backup heating system

[Thaler]

I have upgraded my HVAC with a Lennox 1-stage heat pump system(EL16XP1) with a 2-stage gas furnace (also Lenox EL296UH070XE). The company technician was sure that combination is not supported by my existing Nest learning 3rd Gen. So, so they installed a Honeywell THX9421R thermostat as a solution. After days of googling and discussion with Google tech support, it turns out that my Nest should work just fine. The problem is I can not find (and the online checker tool is not helping me with ) the proper wiring diagram.

The current Honeywell control board is connected with:

R - red

C- black

O/B - orange

AUX - white

AUX2 - brown

Y - yellow

Y2 - blue

G - green

The online checker tool doesn't recognize the AUX2. Can someone suggest a proper wiring configuration with Nest?

[Patrick_Caezza]

Post a picture of your current thermostat's wiring

[Thaler]

Hi Patrick. Do you have any suggestions/thoughts based on my existing wiring?

[Thaler]

Not sure if it would be any different from my description above, but here it is.

[Thaler]

The top two (R and C) wires are additional which are provides power to the Honeywell thermostat screen.

So they're not a part of control wires.

[Thaler]

The more I looked at my current setup (and read about that of course) the more I've confused. The Lenox website clearly described my heat pump as a 1-stage. Moreover, on the PCB inside of the outside unit, there is no connection to second stage (left side on attached photo). So, the heck they were connected the thermostat control board to Y2 wire ?! Does anyone have any idea how I could verify the usage of this connection?

[ChathamHeating]

Y2 out on the outdoor unit isn't hooked up.  the board is designed to work on multiple units.  who ever wired it just hooked everything up without thinking through it.  

[Thaler]

That is exactly my thoughts. 

[Thaler]

So, it looks like the proper wiring should be:

R - red

C- black

O/B - orange

AUX - white

W3 - brown (because it's coming from the backup heat)

Y - yellow

Y2 - blue

G - green

And even while the checker tool now recognizes that configuration, I still have a question how to control 2-nd stage of backup heating?

[Thaler · Accepted solution]

I was able to find the clear answer in this guide http://support-assets.nest.com/images/pro-faq/Nest-Pro-Installer-Guide.pdf

There is not only the wiring diagram for my hardware configuration, but also the clear explanation for my question about how Nest will treat the 2nd stage of heat:

For dual fuel, any wires inserted into the W1, W2/AUX, or Star (if W3 is chosen) connectors will be treated as
alternate heat that will be used instead of the heat pump when needed. Use Pro Setup to indicate the source
(gas, electric, geothermal, etc) as well as the delivery mechanism (forced air, radiant, etc) for each of the W
wires connected. The Nest Thermostat supports two wires for alternate heat; additional alternate heat wires
are not supported

[MelbaDT]

Hey folks, 

Thanks for responding here, Patrick_Caezza and ChathamHeating. 

Thaler, glad to hear that you've found a fix and your thermostat is working as it should with your HVAC system. We also appreciate that you've taken the time to share what you've done as this would be a huge help to others out there who might be experiencing the same. 

Cheers, 

Melba

[MelbaDT]

Hey folks, 

We haven't heard from you in a while so we'll be locking this thread if there is no update within the day. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the Community.

Best,

Melba