Help Identifying a C-Wire

RyanNYC1 · February 3

I’m trying to determine if I have a C-wire in my boiler (Weil McLain EG-65). My Nest thermostat is currently working but recommending that I add a C-wire to prevent potential power problems in the future.

Steamhead · February 3

Is this steam or hot-water?

RyanNYC1 · February 3

Steam

Steamhead · February 3

Post a pic of the left side where the controls are. We need to see what's on the other end of those wires.

RyanNYC1 · February 3

Steamhead · February 3

Thought so. You'll need to create an independent 24v circuit for this, since your limit controls (low water cutoff, pressure control) are cutting off the voltage to the Nest. You can use your existing transformer though, and won't need to alter the factory wiring. Get one of these:

https://www.supplyhouse.com/Resideo-R8222B1067-24V-General-Purpose-Relay-with-SPDT-Switching

Mount it on the inside of the boiler jacket a few inches above the transformer. Connect the thermostat wires on the boiler to the normally open (NO) contacts on the relay. On this one it's the outer two. You'll need some push-on terminals for this.

Connect one side of the relay coil and the common wire in the thermostat cable to the C terminal of the transformer. Connect the red wire in the thermostat cable to the R terminal on the transformer, and the white wire to the other side of the relay coil.

This will provide a constant 24v feed to the Nest whenever the boiler power is on. When the Nest calls for heat, it pulls the relay in, completing the thermostat circuit on the boiler to start the heating cycle.

EdTheHeaterMan · February 3

This comment tells you why you need the relay

EdTheHeaterMan

Jan 12, 2025

The Nest R terminal needs to be connected directly to R terminal of the Transformer. On your boiler the regular 2 wire thermostat is not connected directly to the R of the Transformer. That is because the LWCO control and the Vent Damper MUST be connected to the transformer. There are only 2 wires that can connect directly to the transformer that would the R (where the LWCO is connected) and the C (where the Damper is connected) If you can follow the wiring diagram it is very clear
https://us.v-cdn.net/5021738/uploads/RCB49I69B2BQ/screenshot-2025-01-11-at-8-35-59-e2-80-afpm.png
In order to connect the Nest thermostat you will need to provide another transformer with an isolation relay. This can be a DIY project if you are good with electrical circuitry. If this is over your head, then you will need a professional to get the job done.
If you are up for the DIY method, I can help you thru the steps and parts you will need. First. thing, you need to own and know hw to use a Multi Meter.

Here is the diagram from the comment:

And to add the relay would look like this based on your equipment's diagram

The relay on the top that I added may look like the transformer that is powering the burner, but it is not, The top Transformer.Relay Combination is the additional isolation relay that is needed to properly set up your Nest Thermostat:

Screenshot 2025-02-03 at 12.03.57 PM.png

But your equipment may not be wired according to the original factory diagram, since I see some components that are not original to the equipment that have been added.

Help Identifying a C-Wire

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