Electrical – How to connect a C wire to an Utica PEG112CDE steam boiler

According to the wiring diagram, the transformer should be installed between the T1 and T2 terminals. Though you have not shown the wiring in any of the photos, so it's difficult to give a better description than that.

However, since the transformer is only rated at 40 VA (volt-ampere), the maximum current on the secondary side of the transformer would only be 1.66666667 amperes.

40 VA / 24V = 1.66666666666667 amperes

In most cases the only things on the secondary side of the transformer are switches and relays, so this usually is sufficient. When you go installing thermostats that draw more current than that, you start running into problems.

If you want to install a thermostat that requires constant power, you may have to upgrade the transformer and any fuses that protect it.

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.

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.