Hello all. I want to replace an old Honeywell thermostat with a Nest (3rd gen). I have a old Columbia oil furnace......heat only. Nest support seemed to think their unit was not compatible with this system, so I figured that I would check here to see if anyone had any advice for me before I returned the Nest.

After removing the old thermostat I found 3 wires, blue and white connected to terminals and a red wire not connected to anything. Neither terminal was marked (pic 1).



I followed the wiring down to the furnace. Again the blue and white wires were connected to the controller. The red wire was not used (pics 2 &3).

I'm thinking the white wire controls the heat. Is the blue wire the "C" wire? Will I be able to use the Nest or will I be returning it? Thanks in advance for any help I can get!


Attachment 75864Attachment 75864Attachment 75865Attachment 75866

 

I would isolate the two wires with a relay and provide separate power.
A few options to accomplish this would include using a fan center ( this would be my choice) , an Elk transformer and a 90-340 relay, or a rib relay.

 

Welcome to the forums.

I don't know if Houston was able to see your pictures but there is nothing there.

 

Thanx Pjmax!
I'll try to fix them.

 

Here are the pictures I tried to attach in my first post........I hope this works

 

I was able to see the pictures after logging in.
I didn't see a common available so recommended an isolating relay and separate power source.

 

I went in to the log and the original pic files were gone.
Anyway.... thanks for reposting them.

Houston's method is a little more involved but would be the guaranteed way for the nest to work.
The nest claims to work fine on two wires and technically you could hook the nest to those two wires and have it operate.