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I need to install a nest power connector for my furnace (only equipment present) and am trying to identify the c or common wire. Pic of board and transformer (?) below. Thank you

transformer

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Edit: better pics

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EDIT 2: Testing like this I don’t find 20V on either. Doing something wrong? enter image description here

Edit 3:

By the way this is a honeywell bwbban000120aaaa furnace. Found info on the boiler control unit which does reference “common ground” on the secondary plug. I guess I have a 50-50 shot here http://www.graycoolingman.com/uploads/1/0/6/6/10667336/s9360a_9361a_s9380a_s9381a_boiler_controller.__66-1203.pdf

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  • Your second pic is just a bunch of wires. We can't read the labeling on the board from that image because there's too much stuff in the way. Please edit in a better pic so that people can read the board and include the brand/model of furnace, too.
    – FreeMan
    Commented Nov 16, 2022 at 18:20
  • Updated OP - thx
    – bikeactuary
    Commented Nov 16, 2022 at 21:55

2 Answers 2

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Your low-voltage plug is the one with the pink/brown wire (next to the screw terminals). One of those wires is your common. To figure it out, take a multimeter and probe each wire against your thermostat red wire while the furnace is running. The C wire will give you 24V AC.

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  • Thanks - I must be doing something wrong as I don’t find 24V on either wire terminal while furnace is running. I have the 2 wires twisted together upstairs where the dead thermostat is located (no power, nest) to make this run. See my edit to the OP with a new pic of testing.
    – bikeactuary
    Commented Nov 17, 2022 at 0:09
  • Are you probing with the other lead on the brown or pink wire in the plug?
    – diy_coder
    Commented Nov 17, 2022 at 2:07
  • I tried both, pic shows in the brown. But actually my suspicion is that the pink should be 24v but is not putting out consistent power. This is what the multimeter seems to indicate - I can only get brief bursts of power. Could be intermittent contact w my probe tho. But with the power connector hooked into the pink the nest is back to working with the error code. If I connect to brown then it seems to work entirely off battery and won’t connect to the network. I’m getting low voltage at the therm
    – bikeactuary
    Commented Nov 17, 2022 at 2:57
  • Hmm, out of curiousity what does the multimeter show when you probe R against W (with furnace off)? If you want to confirm your multimeters working right, you can probe pink against brown. That should be a constant 24V AC.
    – diy_coder
    Commented Nov 17, 2022 at 15:11
  • W to R I get a stable 1.5V. Pink to brown is 0 when on standby, a jolt to 1.5 when turning on and then it settles to 0.3V when running. Also when running, W to R bounces around rapidly between 0 and 4V
    – bikeactuary
    Commented Nov 20, 2022 at 15:51
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Page 18 of the manual has your wiring diagram. This is a two-wire control. You connect the wiring to W1 and Rc or Rh on the Nest. If the battery does not charge, you will need to add an external power transformer. Instructions for that are in this video. The short version is that you add a transformer between C and the other R connection to provide power only.

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  • Thx but u haven’t answered my question and in fact repeated things I know and posted in the OP
    – bikeactuary
    Commented Nov 20, 2022 at 21:30
  • I don't see in your original question where you have actually tried connecting the Nest to the two wires that are labeled T-stat on the wiring diagram. Have you done that and it didn't get enough power? If so you could try to find the right power on this board but I would honestly just follow the pattern in that video to add a transformer for the Nest. Even though there's a 24v transformer on the control, without a wiring diagram you're just guessing. That hardly seems worth it to me when an external transformer is a few dollars and will work.
    – KMJ
    Commented Nov 21, 2022 at 7:17
  • Yes - I’m far past connecting my thermostat to the thermostat wires. I’m attempting to locate a stable source of 24V on the board for a nest power connector if you didn’t gather from the post.
    – bikeactuary
    Commented Nov 21, 2022 at 10:42
  • I'm seeing in your other comments that you don't have 24v. There should be 24v across the transformer connector when the transformer is disconnected from the control. If there isn't, the transformer is bad. If there is, I would expect the Nest to be able to get enough power from the two control wires. If it doesn't get enough, you can either add a transformer or try a wire to one of those two Secondary input pins. The second one is a off-label use but I can't see a reason why it wouldn't work.
    – KMJ
    Commented Nov 21, 2022 at 16:26

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