HVAC Thermostat Wire Color Reference: What Every Technician Should Know

The Standard — And Why It's Not Always Followed

The HVAC industry has a widely-used thermostat wire color convention, but it is not a code requirement. Any installer can use any color for any terminal. The convention exists for consistency, not compliance. The result is that a significant percentage of field wiring doesn't match the standard — especially on older installs, DIY wiring jobs, and multi-stage systems where the installer ran out of a particular color.

Never assume a wire's function from its color without verifying at both ends.

Standard Thermostat Wire Colors

R (Red): 24V power from transformer. Some systems split into Rh (heating transformer) and Rc (cooling transformer).
C (Blue or Black): Common — the return side of the 24V circuit. Required for smart thermostats and any thermostat that needs continuous power.
Y (Yellow): Compressor/cooling call. Y1 for single-stage, Y2 for two-stage.
G (Green): Fan relay — calls for indoor blower independently.
W (White): Heating call. W1 for first stage, W2 for second stage or auxiliary heat.
O (Orange): Reversing valve energized in cooling mode (Carrier, Bryant, most brands).
B (Blue): Reversing valve energized in heating mode (Rheem, Ruud, some others) — easily confused with C wire.
E (Brown): Emergency heat.

The B/O Reversal — The Most Common Heat Pump Wiring Mistake

The reversing valve convention is not universal. Most manufacturers energize the reversing valve in cooling mode (O terminal). Rheem and Ruud energize it in heating mode (B terminal). A thermostat wired with O active when B is needed will run the heat pump in cooling mode when the thermostat calls for heat.

Always verify which terminal the thermostat is configured for and match it to the equipment's reversing valve circuit. This is especially important when replacing a thermostat — don't assume the previous installer wired it correctly.

A Fieldmode wire color reference guide — including multi-stage heat pump wiring and common brand-specific exceptions — is available offline at fieldmode.app.