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Please help me find the Common wire. WM gv90+ 5 with wm-odr outdoor reset to use with Nest

shrimpdip Member Posts: 3
edited November 2015 in Thermostats and Controls

I'm a homeowner wanting to install a 3rd generation Nest. Boiler is a WM GV90+5 for two pipe hot water. It was originally a gravity system with Honeywell Unique valves at the rads. I need a c wire to keep the battery charged. The couple places I have called have basically told me to just install a basic battery t-stat and be done with it. They didn't sound like they wanted to mess with it. I'd like something I can access while I'm away from the house.

TT on the boiler control board goes to TT on the outdoor reset. The R and W wires go from outdoor reset to T-stat.
It looks like the c wire should tie into "Br" on the boiler control board. There are no zone valves to deal with. I attached a link to schematic in the manual below.
Link to boiler manual

Outdoor reset manual
Link to odr manual

Am I correct on where to tie in the C wire to the thermostat? I would appreciate any help or advice you can give me.




  • unclejohn
    unclejohn Member Posts: 1,778
    That seems too me as where it would be as well.
  • Paul48
    Paul48 Member Posts: 4,469
    24 VAC Common, right the bottom of the ODR control?
  • shrimpdip
    shrimpdip Member Posts: 3
    Thanks for the replies!

    I checked the 24v Common section on the bottom right of the WM-OCR with a meter and have no voltage coming or going. The common side there appears to connect to chassis ground. I tried connecting the common wire temporarily to the Br wire at the boiler control board and the boiler would just repeatedly start / stop. I might have to check into using a relay and a separate transformer to separate the boiler from the thermostat. You wouldn't think it would be this difficult :-)

    Thanks again for the help.
  • Paul48
    Paul48 Member Posts: 4,469
    Common is like neutral in residential wiring. It is not "hot".
  • shrimpdip
    shrimpdip Member Posts: 3
    edited November 2015
    Well I couldn't make the boiler and the thermostat happy at the same time. I tried tapping into the common on the internal transformer again and the boiler would just quickly cycle on and off. Something on the board doesn't like the nest or the nest doesn't like the board. I reused the separate 24vac transformer from the previous furnace and that powers the Nest and the coil on the relay. The other side of the relay connects where the old thermostat was. So far everything is happy now.

    Thanks again for the advice and helping me think this through.