In a thermostat, the R wire is responsible for powering the entire HVAC system (through a transformer). If you have both an Rh (Red-heat for powering the heating) and an Rc (Red-cool for powering the cooling), then the R wire can be connected to either Rh or Rc. If you only have a single R wire, then it will only have an R connector, which is the connection you’ll use.
Let's Get It Fixed!
HomeX virtual experts will assess your issue (for free!) and can resolve simple problems.