How-to guide
Thermostat Wire Colors Chart (And Why You Shouldn’t Trust It)
By Ismail Farooq · Updated 2026-10-29
Thermostat cable (18/5, 18/8) comes with standard insulation colors, and installers usually follow a pattern. Usually. Here’s the common convention:
| Color | Usual terminal | Job |
|---|---|---|
| Red | R / Rh / Rc | 24 V power |
| White | W / W1 | Heat (aux heat on heat pumps) |
| Yellow | Y / Y1 | Cooling / compressor |
| Green | G | Fan |
| Blue or black | C (sometimes B) | Common (constant power) |
| Orange | O / O/B | Heat pump reversing valve |
| Brown | W2 / AUX / E | 2nd-stage or emergency heat |
Why letters beat colors
Installers run out of colors, reuse cable, or wire a heat pump with a furnace-style cable. White on Y and blue on O/B are both common in real homes. Google, ecobee and Honeywell all tell you to label wires by the terminal letter they came off, not the color. Take a photo before you disconnect anything, then use the wiring tool with the letters.
Where colors most often go wrong
- Heat pumps wired with furnace cable. Orange isn’t always O/B: installers often use whatever color is spare, so the reversing valve can be on blue, black or even white.
- Blue on B. On many older Trane, American Standard and Carrier systems the blue wire is on a terminal labeled B, and B there means common, not the reversing valve. See the B wire page.
- White on a heat pump. On a furnace, white is heat. On a heat pump it is usually the aux heat strips, not the heat pump itself.
- Different colors at each end. The cable to the outdoor unit is spliced at the furnace, so the wire that is yellow at the thermostat may continue as a different color outside.
What to do if a wire has no letter
If the old thermostat’s terminals aren’t labeled, or a wire is loose, don’t guess from its color. Turn the power off and follow the cable to the furnace or air handler: the terminal each color lands on there (R, C, W, Y, G, O/B) is its real job. The old thermostat’s installation manual, found online by its model number, also shows what each terminal does.
Jumper wires
A short piece of wire between two terminals on the old thermostat, most often between Rh and Rc, is a jumper, not a wire from the wall. Don’t count it as one of your wires and don’t move it to the new thermostat. Most smart thermostats join Rh and Rc internally when only one R wire is connected.