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