I installed a nest with 5 wires, but the blue wire is not connected to anything on the 25-30 year old furnace. We tried connecting the blue wire to what we thought was a common terminal, but the furnace started making clicking noises when we turned the power on, and wouldn't respond to Heat tests from the Nest. So we figure we must have connected to the wrong spot. I can't seem to find a wiring diagram, and couldn't find documents online for our furnace.

Currently the nest is connected with just 4 wires (and all is working correctly), but we suspect in April when the furnace is not used, it will run out of power and start causing the furnace to turn on to get power, and the furnace is really old so it might not handle that very well.

These pictures are before we put in the blue wire. The wires from the thermostat are coming in at the top (green yellow red white, red and white wires go left at the top). The place we tried connecting the blue wire is the "C/COM" terminal on the circle-shaped component. Any help would be appreciated. Thanks.

The unit is a Magic-Pak HWC series.. Model number 38HWC122A-10A.

Photo 1 is the full wiring

Photo 2 is the component with the C/Com Terminal

Photo 3 is the component where the red thermostat wire goes to

Photo 4 is the ignition system terminals

Full Wiring Component with C/COM Terminal Component that red thermostat wire connects to Ignition system

  • 1
    Hello, and welcome to Home Improvement. I found the manual here; let us know whether it helps. Feb 25, 2019 at 16:36
  • Wow! I would never have expected it possible! Thanks!
    – Mike E
    Feb 25, 2019 at 17:43

1 Answer 1


Your C wire should be connected the GND terminal on the ignition module.

  • Thanks. I know very little about wiring. I just read here and there that the C wire should be connected to a C terminal (but that an older furnace might not have one). Out of curiousity, do you know what that circular component is, and why what we tried at first wouldn't work?
    – Mike E
    Feb 25, 2019 at 17:45
  • The circular component is a pressure switch. It's a proving device that senses pressure created by the exhaust ventilation system thus proving that there is acceptable draft. When running both lines hooked up to it would be 24 volts. When off one would be 24 and the other one be open line.
    – Joe Fala
    Feb 25, 2019 at 18:03

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