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GV1 - ZONE 1 VALVE GV2 - ZONE 2 VALVE Y1 - YELLOW WIRE 1 Y2 - YELLOW WIRE 2 R1 - RED WIRE 1 R2- RED WIRE 2

TT1 - ZONE 1 THERMOSTAT TT2 - ZONE 2 THERMOSTAT R RED WIRE W WHITE WIRE B BLUE WIRE

BOILER TX TRANSFORMER R RED WIRE B BLUE WIRE G GREEN WIRE W WHITE WIRE

We recently replaced old water boiler heater with new one(williamson) and I took the opportunity to fish new thermostat cable as well(replaced 2 conductor with 5). But my plumber is saying, I still can't use new Wi-Fi thermostat with it and it won't be efficient. I assuming that we wish to finish this job and move on to next one. So can someone please help me to find C wire for Honeywell thermostat. I have attached the current wiring picture with post. If you need any additional details please let me know.

Thanks in advance!

Here are few more pictures.

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In picture you can see green and blue coming to gas control. Yellow is not used and looped back, while white and red is going to outside transformer which is in other side connected to emergency switch.

Can't upload anymore pics to the post. :(

Wiring harness from right side TSTAT DAMPER

Update#3 Boiler make and model http://www.williamson-thermoflo.com/sites/default/files/field-file/gwa_manual.pdf model GWA 175 Valve - Honeywell make

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  • 1
    Can you post detailed photos of the wiring at the boiler please? – ThreePhaseEel Jan 16 '17 at 17:19
  • 1
    BTW: post links to the pics and we can edit them in – ThreePhaseEel Jan 16 '17 at 18:22
  • What's the make and model of the boiler? Can you post the schematic from the boiler (should be inside the access panel)? What's the make and model of the zone valves? – Tester101 Jan 16 '17 at 18:45
  • From what I can tell, you've either not shown us the C wire, or haven't provided enough information about the system for us to be sure which wire would be C. – Tester101 Jan 16 '17 at 19:29
  • What model Honeywell thermostat are you trying to install? – Tester101 Jan 17 '17 at 13:15
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You might be in luck, but it depends on how the Honeywell thermostat works. I know Ecobee3 lets you power the thermostat using the Rc and C terminals, but I'm not sure if Honeywell does the same. However, this solution might still work, even if the Honeywell is not the same as an EcoBee3.

If you look at the documentation for the boiler, you'll see this.

Feild wiring connections for boiler

Which shows that the boiler exposes additional wiring, that's intended specifically for powering a thermostat.

You can see these two wires in the photo of your boiler.

Boiler photo

If the Honeywell can be powered using Rc and C, then here's what you'll have to do.

  1. Install an 18 AWG (at least) 2-wire (at least) cable between the boiler and the thermostat(s).
  2. Connect one of the wires to the wire labeled R in the boiler (as highlighted above).
  3. Connect the other wire to the wire labeled C in the boiler (as highlighted above).
  4. At the thermostat, remove the jumper between R and Rc.
  5. Connect the wire that is connected to the R wire in the boiler, to the Rc terminal on the thermostat base.
  6. Connect the wire that is connected to the C wire in the boiler, to the C terminal on the thermostat base.
  7. Connect the other thermostat wires as they were on the old thermostat.

If the Honeywell thermostat cannot be powered using Rc and C, then you may still be able to get it working. Follow the same procedure as above, but do not connect the wire between the R wire in the boiler and the Rc terminal at the thermostat. Also, leave the jumper in place between R and Rc. This may or may not work, depending on how the system is currently wired.

  • This looks promising. I will follow your instructions and update the thread by weekend. Thank you very much for your detailed analysis! – nil Jan 17 '17 at 19:37
  • Any idea what should be voltage between R and C wire? Before I connect these two to stat, I want to make sure there is enough voltage to power up stat. Thanks – nil Jan 29 '17 at 0:22
  • @nil 24 volts. But it will more likely measure around 27-28 volts with no load. – Tester101 Jan 29 '17 at 1:54
  • You sir, are brilliant! The technician who said it can't be done, you proved him wrong. :) I have connected one of the zone tstat as per your suggestion. It booted up fine and working flawlessly. Now if I have to connect other zone tstat, can I jump second tstat wires at same junction point? I have 2 more additional zone where I am planning to use similar made tstat. Thand! – nil Jan 29 '17 at 3:24
  • @nil you can add more thermostats, however, you'll have been careful not to overload the transformer. I'm not sure how much current smart thermostats draw, but you'll want to check that out before adding more. Otherwise, you'll have to upgrade the transformer. – Tester101 Jan 29 '17 at 15:50

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