I was going to try to upgrade to a smart thermostat but the sites I saw online said thermostats always have an R or RC wire. This is a 1920 house with oil furnace (getting very expensive). The last time I tried to add a smart thermostat to a non-C wire unit at our old place I ended up having to give an HVAC company a whole paycheck.
Not all. RC is power for cooling and RH is power for heating. They are usually bridge together. It’s a 24V AC source.