# On a thermostat, are “R” and “Rh” terminals the same thing?

I have an older thermostat controlling an old ass Trane furnace with A/C. It has an "R" terminal and an "Rc" terminal (they are jumpered - see photo). I see references to an "Rh" terminal online. Are "R" and "Rh" terminals the same thing? This video refers to connecting the other external transformer wire to an "Rh" terminal and I am confused.

# R in general

`R`, `Rh`, and `Rc` are all the same, but not. In general terms, the `R` terminal is where you connect the signal voltage source. In low voltage controlled systems, there will be a step down transformer that provides the power to the control circuitry. One of the legs from the secondary of the transformer will be connected to the `R` terminal, which applies a voltage to the terminal.

When the thermostat wants to be warmed up or cooled down, it closes the appropriate switch, completing a circuit. This allows current to flow from "into" the `R` terminal and "out of" the appropriate signal terminal (`W`, `Y`, etc.) of the thermostat.

# R, Rh, Rc

In some systems, both the heating and cooling systems use their own signal voltage. In these cases, the signal wire from the heating system will be connected to `R` or `Rh`, while the signal wire from the cooling system will be connected to `Rc`.

If only a single signal wire is used, the `R` terminal will be electrically connected (jumpered) to `Rh` and/or `Rc`.

This answer has some diagrams that show how a simple single signal system works. This answer provides another diagram of how a single signal thermostat works, though you can easily imagine how it could be modified to allow multiple signals.

A multiple signal system would look something like this...

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