Wire configuration for dual fuel system?

[bgeis90]

I have a dual fuel system. Current thermostat has 8 wires, Two of the wires are Aux (white) and E (black). Nest doesn't use these wires. Are they no longer necessary? The other wires of existing thermostat are labeled Y (yellow), Y2 (brown), G (green), C (blue), O/B (orange), and RC (red). Is there a wire configuration for this setup? Thank you.

[CoolingWizard · Accepted solution]

@bgeis90 ,

So you are saying you have a gas furnace with a 2-stage Heat Pump? This will require a Nest Learning Thermostat, 3rd generation.  The basic Nest Thermostat cannot support 2-stages of heat and cool nor duel fuel system. 

AC Cooling Wizard

[bgeis90]

Electric furnace is Trane model #TAM7A0C36H31SAA and heat pump is Trane XL16i. If the basic nest doesn't support dual fuel system, then I'll look at purchasing the 3rd generation.

[bgeis90]

Thanks for your help!

[CoolingWizard]

@bgeis90 ,

I looked up your TAM7A0C36H31SAA , a variable speed multi position air handler. This is an extremely efficient unit with optional electric heat strips. Those are controlled by your W1 and E wires. Both wires activate the electric heat strips.  The white wire will be your Auxiliary Heat, this is controlled by the thermostat and the outdoor unit.  Anytime the heat pump is not keeping up with the heat, the thermostat will turn on the auxiliary heat to help.  Additionally, in heat pump applications the home owner has the ability to manually activate the heat strips in the case the heat pump has failed and needs repair.  This ability is the  Emergency Heat function.  
There is an additional control from the outdoor unit, when in the heating mode the outdoor coil will ice up on very cold day/night and defrost control board will activate the auxiliary heat when the out unit goes into defrost mode.  On the Nest Learning Thermostat, your white wire will go to W2/AUX and your black E wire will go to the * connector.  

AC Cooling Wizard

[bgeis90]

Awesome. Thanks for looking it up and providing all the details!