R Wire on a Thermostat: What It Is and Where It Goes

R (sometimes RH, RC, V, or 4 on older units) is the 24 VAC "hot" side of the transformer in your furnace or air handler. The thermostat switches this power out to W, Y and G to turn equipment on.

The wire is usually red, but color is not a rule. Always go by the letter it was connected to. If your old thermostat had a short jumper between Rh and Rc, that is still a single R wire. Don’t connect the jumper to a smart thermostat.

Where it connects on each smart thermostat

Furnace + AC with a C wire

Wires: R, W, Y, G, C · open in the tool

Thermostat	R goes to	Whole setup
Nest Learning (4th gen)	Rc or Rh	Works: direct swap
ecobee Premium	Rc	Works: direct swap
ecobee Enhanced	Rc	Works: direct swap
Honeywell T9	R	Works: direct swap
Amazon Smart Thermostat	R	Works: direct swap

Other wire letters

Rh and Rc Thermostat Wires Explained
What Is a C Wire?
W and W1 Thermostat Wire
W2 and AUX Thermostat Wires
Y, Y1 and Y2 Thermostat Wires
G Wire on a Thermostat
O/B Thermostat Wire
B Wire on a Thermostat
E Terminal on a Thermostat
X and X2 Thermostat Wires Explained
K Terminal on a Thermostat
L, S and T Thermostat Terminals