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Wiring help needed from Honeywell to Nest learning thermostat

Community Member

honeywell thermostat.jpgI currently have a honeywell thermostat installed. I took the picture of wiring. It has 4 wires ,and connected as follows to honeywell.
RC - red color
W- white color
C- green color
K- black color.

Please help me select the right slot in the new nest learning thermostat.
Rc goes into Rc
C goes into C 
W goes into W1 or w2 aux ?
What about the K wire?

Please let me know if first two are correct , and the right terminals for W and K from my old thermostat connection.

Community Member

Dose your system have cooling? Ive been in the a/c and heating industry for almost 40 years and this is the first time I’ve ever seen a K on a tstat sub base.
R goes to RCwith a jummer wire to the rh.

Green goes to G.

White goes to w1

Now  k??? I can only guess that goes to y1 if you have cooling .

If there are any spare wires left you will need to connect one to the common side of the control transformer which is c . If no spares are available then it’s best that you run a new 5 conductor 18 awg tstat cable from tstat location to furnace .

Community Member

Thank you for your response. 

Just to add, yes I have a AC and furnace heating which was controlled with honeywell thermostat. 

Also I did a bit of google search on terminal K, and this is what the info I found on the internet which states " The K terminal located on the Honeywell thermostat combines the Y and G wires. It does this using the Honeywell wire saver." 

Can u figure out now what K is and where will it go on a nest learning thermostat ?

Thanks in advance. 

To be honest this is a tricky one. I’m going to play it safe and suggest that you go ahead and run a new six wire tstat wire form your furnace to the nest .  It might work if you put a jumper from y1 to g but too many things could possible go wrong . Like if you tried the use any of the nest blower functions like post delay it would just keep the condensing unit running. 
its best to bite the bullet and run the new wire.

Hey there everyone, 

I can definitely understand the confusion when it comes to thermostat issues as it can be very odd to figure out sometimes, but I'm happy to look into this for you. Thanks so much to Jeff5462 for the help. 


First off, I would suggest using the Compatibility Checker to see if your system is compatible. Then, if it is, I would suggest contacting a pro to come out to help you with this since I know that the Nest thermostat is not compatible with stranded wires, every wire has to be connected to the Nest Thermostat system. 


Please let me know if you have any further questions or concerns as I'm happy to help.


Best regards, 
Garrett DS

Hey there, 

Just wanted to jump in here to check to make sure that you saw our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours. 


Have a great day. 

Garrett DS

Community Specialist
Community Specialist

Hey all,


I wanted to reach out and let you know that I will be locking the thread at this time. If you have any questions from here, please feel free to create a new post.


Best regards,