There are a lot of things it could be. First, is the outdoor unit
running in heat mode? If so, the big copper line connected to it should
be getting hot. If it's cold, then the unit is running in A/C. Rheem &
RUUD systems have their reversing valves ...
If your Nest isn't detecting any wiring and the furnace breaker and/or
Emergency switch are both on, then the problem will most likely be in
the low voltage wiring. Check the wire coming out of the furnace going
to the thermostat. The plumber may hav...