Need help in replacing Replacing Honeywell TH2110D1009/U thermostat

[SV]

Hello,

I'm trying to upgrade my Honeywell TH2110D1009/U to Nest Learning Thermostat. The current wiring shows "Yellow wire" as disconnected - not sure if I have to connect to the Nest base. Highly appreciate any help!

(Please view the attached pics)

Thx,

SV

[laurentbourg]

Thanks for the detailed pictures. This looks like an electric forced air furnace for heat, no AC, is that correct?

[SV]

Yup, no AC

[sporfrog · Accepted solution]

The Y terminal is used for cooling (A/C). Most often the yellow wire is connected to the Y terminal. If your system does not have cooling, you don't need to use the wire for your Nest Thermostat.  No need to connect it to the Nest base.

[laurentbourg]

Yes the OP first needs to confirm that his HVAC setup does not include any cooling, then we can conclude that the yellow wire is not indeed.

[SV]

Just curious - can I just go ahead and plug in the yellow wire even if I dont have an AC system?

[laurentbourg]

No, you should not plug in the yellow wire. 

[SV]

Additional Reading about the wires that I found on internet:
https://www.aireserv.com/about/blog/2020/february/the-thermostat-wire-color-code-you-need-to-know/

[Patrick_Caezza]

Your current thermostat is compatible with the Nest 3rd Gen Learning Thermostat.

I would suggest wiring it like this.

Red - R to Rh

Blue - C to C

White - W to W1

Green - G to G

Since you have a heat-only system you do not need the yellow wire and it should be left disconnected like it currently is.