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Like many on this site I am trying to install a smart thermostat. I have 3 wires, 24V Red, white, and blue at the thermostat, but only 2 down at the boiler. My boiler is a Burnham Alpine model number ALP080P. I can see where the contractor cut the 3 wires and spliced it with the new wire with electrical tape when we converted to gas from oil. I feel comfortable extending the 3rd wire to the furnace but there is no common, or C, terminal shown where the other 2 wires connect. I have read that I can splice the third wire into a 24V common wire, but I’m not sure which wire would be best. I have included pictures of the wiring diagram and my set up. Any help would be greatly appreciated! enter image description here

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As it stands right now you are not equipped to have a C wire. It is missing. Or you are not telling us all the wires that come out at the thermostat.

Using multimeter find out which 2 wires give you 24 volt.

Or, you will have to run a new C wire from the 24 volt Transformers secondary side to the thermostat

c wire

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    The blue wire at the thermostat isn’t actually connected to it, its just in the bundle and it disappears in the electrical tape near the furnace. Is there a reason I couldn’t use that as a common wire? Running new wires shouldn’t be a problem if necessary but I’m still unsure where the common gets connected at the furnace.
    – Paul
    Oct 19, 2023 at 2:58
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    the OP's problem isn't the lack of a spare wire to use as a C, its that Burnham, in their infinite wisdom, doesn't provide a convenient place on the control diagram to connect a C wire at Oct 19, 2023 at 3:01
  • @Paul you might be in luck, use multimeter and find if you have 24 volt, blue is a good sign.
    – Traveler
    Oct 19, 2023 at 3:06
  • @asinine I’m working on getting a multimeter but I did try connecting the blue wire to the C terminal on the new thermostat but it did not power on, I don’t think it is connected to anything at the other end
    – Paul
    Oct 19, 2023 at 3:19
  • @Paul then that could become your C wire, just connect to the 24 Volt transformer
    – Traveler
    Oct 19, 2023 at 4:43

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