Thermostat Wiring: Terminal Letters and Wire Colors Explained

Thermostat terminals are fairly standard; wire colors are not. Wire by terminal, and confirm at the equipment end before you power up.

Terminal reference

Terminal	Function	Common wire color
R	24 VAC power from the transformer	Red
Rc / Rh	Separate 24 V supplies for cooling and heating (two-transformer systems). Jumpered together on most single-transformer systems	Red
C	24 VAC common (return), needed to power most smart thermostats	Blue or black
W / W1	Heat, first stage	White
W2	Second-stage heat, or auxiliary heat on some heat pumps	Brown or other
Y / Y1	Cooling (compressor), first stage. Also the compressor in heat pump heating	Yellow
Y2	Second-stage compressor	Light blue or other
G	Indoor blower fan	Green
O	Reversing valve energized in cooling (most brands)	Orange
B	Reversing valve energized in heating (traditionally Rheem/Ruud). On some older systems, B is the common, so verify	Dark blue
E	Emergency heat (heat strips only, no compressor)	Varies
AUX	Auxiliary heat	Varies
L	Fault or service indicator from the heat pump	Varies

Conventional system vs. heat pump

• Conventional (furnace + AC): R, C, W, Y, G. A call for heat closes R to W; a call for cooling closes R to Y and R to G.
• Heat pump: R, C, Y, G, O/B, plus W2/AUX/E for strips. Y runs the compressor in both heating and cooling, and O/B decides which mode. Getting O vs. B wrong swaps heating and cooling. See heat pump blowing cold air.

How to verify a hookup

1. Before you disconnect anything, photograph the existing wiring at the thermostat and at the equipment.
2. At the furnace or air handler board, follow each conductor to its terminal. Colors at the thermostat may not match colors at the equipment, especially on older installs.
3. With power on, check 24 V between R and C at the thermostat.
4. Test each call: jumper R to the terminal at the board briefly, or use the thermostat, and confirm the right equipment responds.

Don't short R to C. A short across the transformer secondary will blow the board's low-voltage fuse or damage the transformer.

No C wire?

Options include using a spare conductor in the existing cable, running new cable, or using an add-a-wire kit from the thermostat manufacturer. Avoid "power stealing" setups that make relays chatter or equipment short cycle.

FAQ

What does the C wire do?

It completes the 24 V circuit so a smart thermostat has continuous power. Without it, many thermostats try to steal power through other circuits.

Is the blue wire always C?

No. Blue is often used for C, but it's also used for B or second stages. Always verify by terminal.

Why are R and Rc jumpered?

On systems with a single transformer, the jumper sends the same 24 V to the heating and cooling circuits. Remove it only on systems with two transformers.

Sources

• HVAC School: technical tips and diagnostics