I have an old Lennox boiler which doesn't appear to have a PCB board. Originally the old honeywell thermostat had a 2 pair wire attached which included Red and White Wires. I snaked an 18/5 thermostat wire and replaced the thermostat with a google nest thermostat. When I tried using just the red and white wire it prompts me to attach the C wire, but I'm not sure where to attach it on the boiler. It also prompts me that it detects the white wire, but doesn't detect the red wire. I'm not 100% certain whether it is not detecting it solely because the C wire isn't currently attached. I can confirm that there is definitely power coming from the switch to the boiler. The wires coming from the switch are marked with L1 and L2 and I tested it with a voltmeter. When I test the voltmeter against the white and red wire I get about 26 volts so I'm not sure why the smart thermostat isn't detecting it.

If i'm missing any pertinent information guys please let me know and I'll include it.

boiler wiring pic 2 boiler wiring pic 1


You've identified the mains supply wires feeding the transformer's primary -- the black and the white wires marked L1 and L2 (the latter is actually better labeled neutral). The label on the transformer is visible (PRI 120V).

The two transformer secondary wires aren't visible in the photo. They are two of the three wires that need to go to the thermostat. You should be able to see these secondary wires coming off the transformer and one of them already connected through to the red or the white thermostat wire. The other transformer secondary wire is your C wire.

I have a hunch that your R and W wires may be reversed. R is the one that should go to the transformer's secondary. With the old thermostat the positions of R and W didn't matter, but with the Nest thermostat it does matter. If you do find that W connects to the transformer and R disappears elsewhere then simply swap the two.

Edit: Additional info in a comment confirms the R and W are backwards according to modern convention. Try the following:

  1. Swap the R and W thermostat wires -- R would connect to the yellow transformer secondary; W would connect to a bundle of stuff that can't be seen clearly. Maybe the the white-with-blue-stripes wire?
  2. Confirm that the blue wire and the white wire nutted together are the other terminal of the transformer. Visual check is good; voltmeter check for ~26 volts from these to the R-and-yellow set is even better. Add the C wire into the nut with the blue and the white wires.
  3. It wouldn't hurt to do a few manual checks at the thermostat location before attaching the Nest. Confirm ~26 volts from R to C and from R to W. Probably 0 volts from W to C. Finally try shorting R to W. The system should begin its usual heat cycle and continue as long as R and W are held shorted.
  • Hey Greg I took a look at the back of the transformer, but couldn't get a good angle to take a picture. It looks like it has two wires. Yellow, which is connected to the white thermo wire and blue, which is on the far left of the second picture with the question marks. It connects to a thick white wire which seems to lead to the panel above. The panel above has an amp. – Jonx1000 Nov 10 '20 at 19:24
  • Thanks for clarifying where the transformer was. I connected the C wire to the blue wire and reversed the white thermo wire with the red now its working like a charm. Saved me a couple hundred. – Jonx1000 Nov 10 '20 at 21:26

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