I have a two wire thermostat, red wire and White wire. I have 24 V on the red wire at my thermostat location. I understand the nest needs a C wire. And the C Wire is constant 24 V. Why can I not add a jumper wire from our to see at my thermostat location to give my device constant voltage on the C wire?
Generally speaking the common completes the circuit of 24v so it can be used to power the thermostat. The red wire in a two wire system is not a replacement for C (common). Having a C wire is recommended for a Nest thermostat. You might be able to get a Nest Learning thermostat to work in your situation. Don't jumper R to C.