This image is from another board that I obtained for a heat pump system.  It shows the c-wire connectionsMy existing Nest learning thermostat, 3rd gen, is draining the battery really fast when use the wifi. I have a ClimateMaster Water Source Heat Pump. It has only four wires. I run an extra wire to use as C-wire, as I could not find common wiring in the CXM controller. Now I need to connect the c-wire to the blue common wire coming of the 24 Volt side of the transformer.

I would love someone to triple check the way to tap into the common wire:

I can use an adapter to piggyback the blue wire located just above the yellow wire at the board and from there connect it to the C terminal - and the other side would be connected to the nest thermostat.

I am attaching some photos with the existing wiring.

Any help would be appreciated. enter image description here

  • 1
    There's only one picture and it's unclear what you are trying to show. Did you run another wire? Does the above picture show the original wiring or have you made changes? The cable that contains the red, white, blue, and green wires - are you sure there's not another wire in the cable that might have been cut off at the ends?
    – JPhi1618
    Oct 20, 2021 at 15:32
  • There's a green screw-down connector block on the left and a white push in connector block on the right. Which one are you trying to connect the wire to? Which terminal on the block are you planning on using?
    – FreeMan
    Oct 20, 2021 at 16:40
  • I am trying to connect to the green screw-down connector block. I am planning using the C terminal on that green block. The current connections on that terminal are: Y (compressor), O (heat pump), G (fan) and R (cooling). I have run a 18 gauge green wire from the thermostat to the control panel to use as the c-wire, because the current thermostat cable has no 4 wires. There no other wire in the cable that might have been cut off at the ends. The green 18-gauge wire has not been connected yet to either the Thermostat or the board. I am just checking with you experts first. Oct 21, 2021 at 16:14
  • BTW, the image shows the original wiring from 2016 when the heat pump and the Nest Thermostat were installed. No changes have been made, other then running the new green 18-gauge wire to be used as a potential c-wire. Oct 21, 2021 at 16:20


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