Adding C wire to Nest thermostat

Tolik

I've had the 2nd gen Nest for many years with no C wire and it works more or less.

I had some renovation done which allowed me to run a five strand wire from thermostat to near boiler.

Google has decided that the 1st and 2nd gen Nest no longer deserve updates after October 2025 so I may want a newer one (which they claim will be offered at 50% off). The 4th gen has some "features" that only work if there is constant power.

I tried to figure out how to add another wire for C but fell short. Any advice would be greatly appreciated.

I drew out a diagram (best effort) of the current wiring situation. As well as made a short video for reference. https://www.youtube.com/shorts/k3mUnmN__u8

EdTheHeaterMan

Here is a rough transcript from the video.

So I ran a new cable from the thermostat five strand. These are the original two cables (Red and White) . These are the original ones that went to the boiler , this is another one I just spliced together (Blue) for now .  But these are the only two cables that the thermostat had. The red cable becomes green and goes into the low water cut off (LWCO).Right there and that’s it.

From the LWCO the other cables come back into the transformer.  

The white (from the thermostat) goes here and ties into the green, goes around and goes into a pressuretrol.  Then it comes back and goes here then goes into another pressuretrol and from this pressuretrol we got cables coming back and it goes into the flue control I think it’s called (Automatic vent damper).

And from there it comes back here, a whole bunch of them. And go here (indicating the two wires on the transformer) to this transformer and the other one comes down and goes into black solid and goes into 24 ground on the gas valve control.

Here is the wiring diagram that explains that video:

• The Green shaded area corresponds to the portion of the transcript indicated by the letter A
• The Red shaded area corresponds to the portion of the transcript indicated by the letter B
• The Blue shaded area corresponds to the portion of the transcript indicated by the letter C
• The Orange shaded area corresponds to the portion of the transcript indicated by the letter D

I did this to make it clear that all the wires you have in the video are already on this diagram.  I need you to see that the Nest or any smart thermostat that requires a C wire will not work on your steam boiler with the original wiring diagram.  That is because one of the thermostat wires (The red or the white) MUST be connected directly to the transformer.  If you look at your wiring diagram, the red wire connects to the LWCO And the white wire connects to a pressure control.  Neither of the original thermostat wires are connected directly to the transformer. 

This is a real problem for a lot of people with steam boilers who want to connect a smart thermostat that requires a C wire. There is a way to do it if you use an isolation relay and a separate transformer for the smart thermostat to operate that relay.

EBEBRATT-Ed

RIB RELAY & TRANSFORMER FOR NEST.pdf

EdTheHeaterMan

With any smart thermostat you MUST connect 24 VAC power to the thermostat by way of the R on the transformer to R on the thermostat and C on the transformer to C on the thermostat. That is impossible on this diagram because R from the thermostat DOES NOT get connected directly to the R on the transformer. There is an operating or safety limit control of some type between R on the transformer and R on the thermostat.

Here is the isolation relay that you will need. This is the most inexpensive relay with transformer you can purchase. Others may recommend the RIBU1C relay and a separate transformer. And the cost can be about the same. But I find this one to be dependable and easy to understand.

Here is the wiring diagram for you to use with the Isolation relay.

There are 6 wires from the back side of the relay and 2 wires from the transformer. Connect the black and white from the transformer to 120 VAC. Connect the Purple and Red/Yel wire (from the 6 relay wires) as shown to the location where the thermostat used to go. Cap all unused relay wires separate with separate wire nuts so stray voltage from unused wired does not cause a short circuit.