C is a common neutral wire. You can't jump it from Rh as that is a 24v wire.
You need a third wire, C, to return to the unit and hook to the common side of the transformer.
|reply to Nazdak76 |
For ease of thought, think of R as "hot" and C as "ground". If you jump them together you create a short circuit. You'll trip the low voltage breaker/fuse in the furnace if it has one, or fry the transformer if it does not. You don't want to do this.
Your two wire system used a passive, non-powered thermostat. The new one you have needs to be powered by the system to operate. This is why it needs that common wire going back to the system, to complete the power circuit.
Edit: Well, concerning the first paragraph, if you don't have have a C wire going back to the system, then there will be no short. But the thermostat will not have a complete circuit for power. You need to somehow get that third wire from the heating system to the thermostat.