I have an old gas furnace/boiler for hot water baseboard heat and I'm trying to switch from a 2 wire mercury bulb thermostat to a Nest Thermostat "E" (the new white faced one).

There are 5 wires in the wire bundle between the furnace and the thermostat with only the red and white being used in the original setup. I believe I've added a common wire correctly by connecting one of the extra wires to the "C" terminal on the transformer (see pictures below) and the other end to the "C" plug in the Nest.

The Nest powers up fine and seems to detect the wires without any errors. But it's not able to switch on the heat. If I turn the heat up all the way and wait the screen turns orange (to indicate that it's heating) but the furnace never switches on.

some things I've checked

  • If I unplug the R and W wires from the Nest base plate and touch them together the heat immediately switches on. So I don't think I've broken anything.
  • I have a cheap voltage measurement tool that reads 24v touching one end to R and the other end to W at the thermostat base plate. It also reads 24v touching one end to R and the other to C. Is this correct?

Anything else I can try? I'm not sure if this is an issue with the nest or with my wiring. I gave up on Nest tech support after being on hold for 30mins. and I figured I'd check the internet before Nest tells me to call a pro.


Let me know if you need more info or pictures

Original setup behind the thermostat: enter image description here

Original setup at the furnace/transformer: enter image description here

This is how the boiler is set up enter image description here

close up of original connections on transformer enter image description here There are 5 connection points on the card, I'm assuming the R (bottom left) and C (top left) are the only ones actually attached to the transformer. The middle one (white circle) I think is labeled "G".

transformer connections with blue common wire connected under the screw enter image description here

So my common wire is going directly from the "C" terminal on the transformer to the thermostat. Everything else is unchanged.

Nest wiring screen enter image description here

  • Can you get your paws on a multimeter and put it between G and C, set for volts, while the Nest is trying to call for heat? Dec 7, 2017 at 0:45
  • I don't have an actual multimeter, but using this thing I get no voltage measured between G and C while the Nest is calling for heat.
    – Tam Poivre
    Dec 7, 2017 at 1:06
  • What voltage do you measure between G and R while the Nest is calling for heat, for that matter? Dec 7, 2017 at 1:09
  • Between G and R I get 24v. But I think it's 24v regardless of what the Nest is set to.
    – Tam Poivre
    Dec 7, 2017 at 1:18

1 Answer 1


Broken Nest

Either the output driver for the W output on your Nest is toast, or the Nest's solid-state output drivers can't put out enough current to drive the load of your system. Try switching the wire connecting the Nest's W output to the Nest's G output and seeing if turning the fan on causes the boiler to fire up. If it doesn't, then you can't use the Nest with your setup, sorry!

  • I tried moving the W wire to G on the nest and turning on the fan, and still nothing happened. Unfortunately I think it's not going to work with my setup. Thanks for your help!
    – Tam Poivre
    Dec 7, 2017 at 15:11

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