Hello,

I am looking at adding a zwave thermostat and want to power it with the spare wire that is not connected at either end. Currently I am using the standard 4 wires. I have opened up my furnace and noticed the common terminal has a wire connected to it, upon tracing it down I found a junction box where the thermostat wires connect up with twist caps. There is a third cable (2 wires red and white) attached in that heads out to my compressor, It is connected to the blue common wire from the furnace controller and spliced into the yellow wire from the thermostat and furnace. Is it ok to use the spare wire in the thermostat bundle to power my c wire to the thermostat, or will i damage my compressor?

Thanks

 

It is normal to find 2 wires on common.