Nest thermostat not turning on heat

boilerheatingnestthermostat-c-wire

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.

PICTURES

Let me know if you need more info or pictures

Original setup behind the thermostat:
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Original setup at the furnace/transformer:
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This is how the boiler is set up
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close up of original connections on transformer
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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
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So my common wire is going directly from the "C" terminal on the transformer to the thermostat. Everything else is unchanged.

Nest wiring screen
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Best Answer

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!

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