A contractor is replacing my old bryant system with a trane XR15-XV80 system. The existing thermostat wire has two conductors to the outdoor unit, but the trane system has two stage cooling and requires three wires. The existing wire is run through a two story wall to the attic so probably cannot fish a new wire. The contractor gave me two choices:
1. Run a new 3 conductor wire along the outside of the brick, through the eave and into the attic.
2. Use the existing two wire conductor and use the the copper refrigerant line as a third wire.
Option #2 looks better, but it sounds like a hack. Any suggestions?