Wiring Inside Boiler - C Wire Question

You have a classic problem that steam boilers present. The C wire is unable to be connected without an isolation relay. If you had a good technician that understood how control system circuitry operated then there is a rewire that will work, but technicians like that are hard to find. The best fix is the isolation relay.

Can you show us the model number and or the wiring diagram from the boiler? It is usually on the front door panel

Here is a post from a while back that addresses your issue:

You may need to click on some of the links to open the diagrams

As i mentioned above, you are running into a common wiring issue. The Nest thermostat needs a direct connection to the boiler’s power source. In your system, both the low-water cutoff and the automatic vent damper also need their own direct power connections. Because the transformer only has two terminals, there isn’t a spare, uninterrupted connection available at the thermostat terminals.

As a result, the Y or G terminals cannot be used to power the Nest. There is simply no reliable way to add a C wire without it passing through either the low-water cutoff or the vent damper. If either of those safety devices activates, it would interrupt power and the Nest would stop working.

The easiest way to get this to work is to use an isolation relay for Nest thermostat to operate with a direct connection to a transformer and the relay contacts to do the thermostat connection to Y and G

Diagram to follow

Unless something has changed, all you need to do in order to get a C wire from that boiler is connect it to the boiler jacket. Mine is connected with the ground wire to one of the burner igniter control box mounting screws

I did change my wiring so the LWCO comes after the thermostat, but that's optional IMO especially with a Nest that has a battery.

Yes Paul, that really is a concern. Actually the Vent damper cuts power to the thermostat C on every call for heat being satisfied. That is not a failure of the vent damper, that is normal operation. So normal operation will cut power to the thermostat C terminal as a course of normal operation. So as a course of normal operation to the Nest will fail as a result of lost power every time the damper closes. That is really a concern!

I remember getting an error on my own Nest when the vent damper on a water boiler would interrupted the power at the end of each cycle. At that point I would need to go to the thermostat to reset the "Loss of Power" error in order to get the thermostat back online. Sometimes it would reset automatically but many times it needed human intervention. Not a good fit for any situation.

Same holds true on the LWCO side. As a course of normal operation the R terminal on the thermostat will loose power. Steam boilers often go off by way of the cycle guard or actual low water condition regularly. That is not a failure of the LWCO, but normal operation. That normal operation will cause a power failure to the R on the NEST and that will see that as a failure, not a normal thing that just happens. That also is a real concern

The Nest or any smart thermostat needs to have power from R all the way back to C for normal operation without interruption. The operating controls interrupt as a part of normal operation. So the best action is to put the Nest on an isolation relay where the R and C are not interrupted and use a relay to open and close the T T leads of the control system as shown here. (T T is actually Y and G on this appliance)

Easiest fix in my opinion.

It seemed like you changed your story, Ed 😅

I was responding to what you originally said:

If either of those safety devices activates, it would interrupt power and the Nest would stop working.

But regardless, it's pretty easy to test running a wire to C and then C how it goes

You're 100% correct on that.

I changed mine so the LWCO comes after the thermostat.

However, I feel with a Nest it probably wouldn't matter due to the internal battery. An assumption on my part.

In a perfect world someone will be feeding the boiler normally anyway leaving the feeder only for emergencies. I know, but I can hope.