Where do I connect my C wire from my thermostat when there are two transformers?

Question

I have a Laars gas boiler that has 3 heating zones. I want to replace the old mercury honeywell tstat on the 1st floor (heat only) with a newer honeywell RTH9580 wifi model. I've taken pictures of my entire system, labeled the wires in the pics to indicate the associated connections, and included the Laars wiring diagram. There is a transformer mounted on the wall near the boiler that is wired in-line to the system (power from the panel --> into the emergency boiler switch --> into the wall transformer --> then to the boiler (low water cut off and boiler circuitry). There is also a transformer within the boiler circuitry itself. Can someone tell me the best method (proper preferred, not necessarily easiest) to connect the C wire?

Answer 1

Okay... After reviewing your new photos, I think I have it figured out.

This is what your system looks like

^{You'll have to excuse the glitter pens, it's the only thing I could find on my wife's desk.}

It's pretty simple actually. When a thermostat calls for heat, the valve for that zone opens, and the boiler is also signaled via the auxiliary switch in the valve.

To get a C wire to one or more of the thermostats, you'll simply run a wire from the other side of the transformer on the wall.

^{NOTE: If this diagram is accurate, the white wire to the thermostat is R, while the red wire is W.}

The original wire from the transformer should be connected to the R terminal on the thermostat, while the new C wire should be connected to the C terminal on the new thermostat. This should supply power to the thermostat, and allow it to operate as normal.

The way your thermostat is currently wired, you'll have to swap the white and red wires to get it to work. So white will go to R, and red will go to W.

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To make this fit a more common wiring style, I'd probably rewire it a bit. So that the red wires going to the thermostat, connected the transformer and the R terminals. And the white wires connected from the zone valves to the W terminals on the thermostats.

^{Rewired to fit a more common style.}

Then you'll run the blue wire from the white transformer lead, up to the C terminal on the thermostat.

Answer 2

For others looking at this post years later...I just spent a night researching this. Seems like this worked for the OP, but watch out on steam or other gravity fed systems. For a simple single zone steam system at least, the thermostat switch is just another switch in line (in series) with all the other safety cutoff switches and the pressuretrol as well. What that means is that

1) your R wire might not always be hot for this to work. On my Utica boiler, for example, the pressuretrol is between the 24vac power and the R wire. So my R is only hot when the heat is on (not helpful to power a thermostat to turn it on..but if a catch 22)

2) Even if it is, it could draw current through other things such as an electric damper or even the gas valve if it's really stupidly wired in order to power your fancy Wi-Fi thermostat. That causes problems for those devices AND causes a voltage drop at the thermostat (although I suspect it can handle a bit as it likely regulates that down anyway).

All that said, the right way on these systems where the thermostat switch is hard wired is to install an isolation relay. Ecobee had a good FAQ on that here: https://support.ecobee.com/hc/en-us/articles/227874527-ecobee3-with-add-in-Isolation-relay

You also may be able to run a 4th wire with the constant power if you're heat only and split Rh and Rc. Emerson has an article about that using a wall wart, but same concept would apply to running a fourth wire straight to the transformer on the boiler. Not sure if this is common for most Wi-Fi thermostats or not...it relies on then sourcing power from Rc only and Rh just being a dumb switch. https://sensicomfort.com/support/article/adding-a-24-vac-external-transformer