Two Wires to Aux/E on Honeywell. 7 Wires Total. Did I Buy the Wrong Nest Thermostat?

[Jeff_Oregon]

Existing thermostat has seven wires.  Two wires (white and black) to Aux/E (not a separate Aux and separate E).  Five regular wires to R, Y, O/B, C & G.

Purchased the basic Nest; not the learning nor the E.  Six ports.

Heat pump with heat-strip backup.

Current system can turn on emergency heat, which runs strips but not the heat pump.  Auxiliary kicks the strips on to support the pump when the pump can't keep up.  Don't know which is the black wire and which is the white.

Is the basic nest capable of providing the current functionality?  I'm thinking not.

Would the learning nest provide the current functionality?

 

[CoolingWizard]

@Jeff_Oregon , your old thermostat combined Auxiliary Heat and emergency heat. You have to use a Nest Learning thermostat if you want to have Emergency heat capability. 

AC Cooling Wizard

[Jeff_Oregon]

Thanks!  I presume I'll need to get into the wiring at the other end to confirm which wire is running which functionality?  Is there a typical standard color coding for auxiliary and emergency?

[CoolingWizard]

@Jeff_Oregon , the Thermostats that are heat pump smart, provide both Auxiliary Heat and Emergency Heat. The concept is the owner need an ability to turn on the electric heat strips manually. If for example, your heat pump system fails and needs service, you the home owner can turn on the electric heat while you wait for the HVAC service company to arrive.  By contrast, the auxiliary heat is under the control of the Thermostat or the Heat Pump Defrost Control Board.  In this case, if the heat pump goes into defrost mode, the auxiliary heat, if present, will be activated.   In a second example, of the thermostat sees that the heat pump is not keeping up, it will activate the auxiliary heat as a booster.  

AC Cooling Wizard

[Jeff_Oregon]

Thanks so much.

I did a bit more investigating on how the Trane heat pump / air handler / existing Honeywell thermostat is wired.  Pics attached.

On the thermostat, both a black and white wire are attached to a single AUX/E terminal.

Both the black and white wires are connected to W2 in the air handler.  The white wire then continues and also connects to W1.  The black wire is only on W2.

If the AUX/E is energized, W2 and W1 would seem to be engaged just off the white wire.  Is the black wire needed? 

Appreciate guidance on which nest thermostat will work best with the system, as well as how to wire the black and white wires at the thermostat.  And if changes are needed at the air handler?

Thanks again!

[EmersonB]

Hello everyone,

@Jeff_Oregon, thanks for reaching out here in the Community. To know more about this issue, I’d like to confirm, where are you located? Were you able to buy the Nest Learning Thermostat? The NLT 3rd gen has 10 terminals that can accommodate the wires like your system. Let us know if you have more questions in mind.

I appreciate your response, CoolingWizard.

Regards,

Emerson

[Jeff_Oregon]

Have an older Trane heat pump and intending to replace the Honeywell thermostat with a smart Nest.

Seven wires to the thermostat.  Five make sense (R, Y, C, O/B, G) but a white AND a black wire are on a single AUX/E terminal.

In the control box at the air handler, the white wire goes to W2 first, and then on to W1.  The black wire goes only to W2.  The control box is also used as a junction box, connecting same colored wires from thermostat to the control box and from the control box to the heat pump.

If AUX/E is energized it would seem to call for both W1 and W2 from the white wire.  What purpose does the black wire serve, since W2 is already energized off the white wire?  The current thermostat offers Auto, Cool, Heat, and Em Heat system options.

I understand the difference between auxiliary heat (with heat strips coming on in super cold conditions where the heat pump can't keep up) and emergency heat (with the heat pump shut off and relying solely on the heat strips).  And I want to keep those capabilities, and having that control from the thermostat and the app.

The system has worked fine for years, and I'm looking to simply add wifi control with the new thermostat.

Seems the basic Nest I bought won't work, and the learning Nest is needed?  And seems I may need to make dedicated connections between W1 in the thermostat to only W1 in the control box, and a dedicated connection between W2 in the learning thermostat and W2 in the control box?

Or is it time to hire a pro?

Thanks for your help and education!

Current Control Box WiringCurrent Thermostat Wiring

[Markjosephp]

Hello Jeff_Oregon,

I've duplicated your thread in the main one here. You’ll receive all updates on this issue on the main thread, and we recommend updating and checking this thread when needed.

Regards,

Mark