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I recently converted from Oil to Gas. One of my zones is no longer working post conversion (it was working, then nothing worked, and then after they replaced a defective unit on the boiler it didn't work but other zones worked). The plumber said it's most likely the wire between thermostat and the and the system. Here's a picture of what the thermostat set up is:

Nest set up

Using a voltmeter I can measure ~28V between Rc and G and Y1 but only get roughly 16V (slightly under) for W1 and Rc. What could be causing this this voltage drop? On the other two thermostats the Rh and W1 wires read ~28V. Here's what the wiring looks like at the boiler for that zone:

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You can see that there's an extra wire that's covered in tape that's coincidentally very close in color to the W1 wire. It's also the only zone that has that extra wire. All other zone do not. When I do a volt reading between the blue and yellow capped wires in the picture, I get ~28V when the thermastat says heat. When I do it between the taped and the yellow I get roughly ~58V. What is that wire? Should I wire it in? Is it part of the problem?

Edit more picture based on question below: enter image description here enter image description here enter image description here

  • You're going to have to provide us with a detailed schematic of the control wiring for the system. Is there a zone controller involved, or is the system simply using zone valves? It's possible that the boiler control system is using a different transformer. Without knowing a lot more about the system, there's no way we can answer definitively. – Tester101 Oct 3 '17 at 12:17
  • I added more pictures. I'm assuming the "zone controller" is the green "zone switching" relay in the pictures. I opened to box and took a picture inside too. The wiring from what I can tell is wires from the thermastats go to the basement and then get wire capped to another wire that go down to the controller. Except for that zone which has that extra uncapped wire that I don't know where it's from or where it goes. Its possible there's a second transformer for the boiler. How would find out? What other information would help? – Irina M. Oct 5 '17 at 2:56
  • Great, thanks. Now... The wire that's connected to the Rc terminal on the thermostat, does that come from the boiler zone controller, or the A/C air handler? – Tester101 Oct 5 '17 at 14:51
  • If you have separate heating and cooling systems (like it appears you do), there should be two R wires. Rc which is power from the A/C control transformer, and Rh which is power from the heating system control transformer (in your case the zone controller transformer). – Tester101 Oct 5 '17 at 14:54
  • I believe it comes from the AC air handler because when they were doing the switch over, that was the only Nest that didn't lose power and could still trigger the AC (located outside). If I unplug my AC air handler, that should report 0 voltage, right? I don't think there was ever an Rh wire. – Irina M. Oct 6 '17 at 1:43

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