Trying to replace an old round thermostat with a new noma digital. Instructions ask to connect the R wire with the Rc wire but old thermostat only has an rh wire and r wire. Is the rh the same as the rc wire?

Rh is the feed for the heating control, Rc is the feed for the cooling. Sometimes a heating plant will have its own power source for control, and the cooling will have its own.

Most times one source can power both control circuits, and Rh and Rc can be jumped out so one source can feed both control circuits.

I will also copy this over at HVAC forum to get help fron the resident experts there that deal with this more than I do.

Exact model and brand of old thermostat?