SAFETY: Turn off power at the breaker or the furnace switch before touching any thermostat wiring. Confirm the thermostat display is dark before pulling it off the wall.
Thermostat wire colors are conventions, not rules: red is usually R (24V power), white is usually W (heat), yellow is Y (cooling), green is G (fan), and blue is usually C (common), but the installer may have used any color for anything. Always match the terminal letter on the old thermostat, not the wire color. Here’s the master chart, then the per-color breakdown with the multimeter check for each.
| Wire color (typical) | Terminal | What it does |
|---|---|---|
| Red | R (or Rc / Rh) | 24 V AC power from the transformer. Rc = cooling power, Rh = heating power; one system uses a jumper between them. |
| White | W (W1, W2) | Heat call. W2 is second-stage or auxiliary heat. |
| Yellow | Y (Y1, Y2) | Cooling call — energizes the compressor contactor. On heat pumps, Y runs the compressor for heating too. |
| Green | G | Indoor fan (blower) relay. |
| Blue | C | Common — completes the 24 V circuit and gives smart thermostats constant power. Sometimes black. |
| Orange | O (or O/B) | Heat-pump reversing valve, energized in cooling (most brands). |
| Dark blue | B | Heat-pump reversing valve, energized in heating (Rheem/Ruud). Occasionally used as C — verify. |
| Brown | E | Emergency heat — manual backup that locks out the compressor. |
| Varies | AUX | Auxiliary heat — automatic electric backup on heat pumps. Often shares a wire with W2. |
This page decodes the colors. For terminal-by-terminal wiring and the full replacement procedure, use the thermostat wiring guide; for every printable schematic in one place, the thermostat wiring diagrams page. Photograph your old wiring before disconnecting anything, and label each wire with masking tape marked with its terminal letter.
Red wire: R, Rc, or Rh (24V power)
What it usually is: the 24V AC feed from the transformer in your furnace or air handler. Every function the thermostat performs is this wire being connected to another terminal.
What else it might be: on systems with separate heating and cooling transformers, you’ll see two red-family wires: one for Rh (heating power) and one for Rc (cooling power). Sometimes both are plain red; sometimes one has a stripe. On some older heat-pump cable runs, a red wire was used for O/B, which is why the terminal letter matters more than the color.
How to verify: with power on and the thermostat off the wall, set a multimeter to AC volts and measure from the suspected R wire to C. A healthy reading is roughly 24 to 28V AC. If you have two red wires, measure between them: more than about 12V AC means two separate out-of-phase transformers, and they must never be jumpered. That situation has its own guide: Rc/Rh jumper wiring.
White wire: W or W2 (heat)
What it usually is: the heat call. When the thermostat connects R to W, the furnace or boiler fires. W2 is second-stage heat on two-stage furnaces.
What else it might be: on heat pumps, white often lands on AUX or W2 (electric backup heat), not primary heat. Primary heat on a heat pump is Y plus the reversing valve, so a white wire there does something different than it does on a furnace.
How to verify: with the system powered and calling for heat, measure from W to C: you should read ~24V AC while the call is active. Or trace it at the furnace control board: the wire landed on the board’s W terminal is your heat call regardless of its color.
Yellow wire: Y or Y2 (cooling / compressor)
What it usually is: the compressor call. R connected to Y starts the outdoor unit. Y2 is second-stage cooling on two-stage systems.
What else it might be: on a heat pump, Y runs the compressor for both heating and cooling. The O/B wire decides direction. So “yellow = AC” is only true on conventional systems.
How to verify: at the furnace board or air handler, the Y terminal wire runs out to the condenser contactor. With a cooling call active, Y to C reads ~24V AC. If cooling never starts after a rewire, a loose Y is the first suspect.
Green wire: G (fan)
What it usually is: the indoor blower relay. R to G runs the fan without a heat or cool call, which is what the FAN ON setting does.
What else it might be: on some older 3-wire furnace installs, the third wire lands on G instead of C. If your thermostat runs the fan but a smart thermostat won’t power up, that third wire is fan, not common.
How to verify: flip the fan setting from AUTO to ON. If the blower starts within a few seconds, G is doing its job. With a meter: G to C reads ~24V AC while the fan call is active.
Blue wire: usually C, sometimes B (the trap)
What it usually is: the common wire. C completes the 24V AC circuit so the thermostat has constant power. Smart thermostats care about this wire more than any other; without it they steal power or run on batteries.
What else it might be: B, the heating-mode reversing valve terminal used by Rheem and Ruud heat pumps. Google Nest’s own wire identifier documentation calls this out: a blue wire can be C or B depending on the system, and you must check the terminal label on the old thermostat rather than assume. Landing a B wire on C (or vice versa) is the classic wrong-blue mistake, and it’s why a Nest that “worked fine for a week” starts rebooting or a Rheem heat pump blows cold on heat.
How to verify: check which terminal the blue wire occupied on the old thermostat. If it was on C, measure R to blue: a constant 24 to 28V AC whether or not any call is active confirms common. If it was on B and you have a Rheem or Ruud heat pump, it’s the reversing valve. The full decision tree, including using a spare blue as a new C wire for a Nest, is in can I use the Nest blue wire as a C wire. No C wire at all? Your options are covered in how to install a thermostat without a C wire.
Orange wire: O (reversing valve, cooling)
What it usually is: the heat-pump reversing valve, energized in cooling mode. Most brands (Carrier, Trane, Goodman, Lennox) use O logic.
What else it might be: if you don’t have a heat pump, an orange wire is a spare or an installer improvisation; check where it landed. On some installs orange feeds W2 or an accessory.
How to verify: you don’t rewire O/B behavior, you set it. During thermostat setup, choose O if cooling energizes the valve (most brands) or B for Rheem/Ruud. The tell for a wrong setting is unmistakable: the system heats when you ask for cooling. Fix it in the installer menu, not at the terminals.
Dark blue wire: B (reversing valve, heating)
What it usually is: on Rheem and Ruud heat pumps, the reversing valve energized in heating mode. Installers sometimes use a darker blue to distinguish B from a light-blue C, but that shade convention is not reliable.
What else it might be: C. This is the same ambiguity as the blue wire above, approached from the other side.
How to verify: terminal label first. If the old thermostat shows this wire on B and your outdoor unit is a Rheem or Ruud, it’s the reversing valve. A constant 24 to 28V AC read against R with no call active points to C instead; B only shows voltage against C when the valve is energized.
Brown wire: E (emergency heat)
What it usually is: emergency heat on heat pumps with backup strips. E is the manual switch that runs the strips instead of the compressor. Use it when the heat pump itself is broken, not just because it’s cold outside.
What else it might be: brown is one of the loosest conventions. On 8-conductor cable it may be a spare, an accessory feed, or Y2. Verify at both ends before trusting it.
How to verify: trace it to the air handler terminal strip. If it lands on E there, it’s emergency heat. Selecting EM HEAT on the thermostat should bring on the strips (supply air gets warm) without the outdoor unit starting.
Color codes by wire count
5-wire thermostat color code
The modern standard: red to R, white to W, yellow to Y, green to G, blue to C. Every smart thermostat installs on this with no adapters.
4-wire thermostat color code
Red to R, white to W, yellow to Y, green to G, and no C wire. This is the classic pre-smart-thermostat setup. Everything works except constant thermostat power, so a Nest power-steals and an ecobee needs its included Power Extender Kit. Check the wall cavity first: many 4-wire installs used 5-conductor cable and tucked the unused blue behind the plate, which is a free C wire.
Heat pump thermostat color code
Red to R, yellow to Y (compressor, both modes), green to G, blue or black to C, orange to O or dark blue to B (reversing valve), white or brown to AUX/W2 and E (backup heat).
The single most important heat-pump check is O versus B. O energizes the reversing valve in cooling on most brands; B energizes it in heating on Rheem and Ruud. Set it in the installer menu during setup.
Honeywell thermostat wiring color code
Honeywell thermostats follow the same universal terminal letters as everyone else, and Honeywell’s own support documentation says wire color conventions vary by installer and system. So there is no separate “Honeywell color code”: red to R, white to W, yellow to Y, green to G, blue or black to C, orange to O. What is Honeywell-specific is hardware like the T10 Pro’s R slider tab (its physical Rc/Rh jumper) and the terminal layout on each sub-base. Match letters and you can move from a 30-year-old Honeywell round to a T9 without a chart.
When the colors make no sense at all
Some installs reuse whatever cable was in the wall: doorbell wire, sprinkler wire, a rainbow that matches nothing. The recovery procedure is always the same:
- Kill power at the breaker. Confirm the display is dark.
- Photograph the old thermostat’s terminals with the wires still landed.
- Label each wire with tape marked with its terminal letter, then disconnect.
- If the old thermostat is already gone, identify wires at the furnace control board end instead: the board terminals are lettered, and a helper at each end with a multimeter (continuity mode, power off) maps each conductor in minutes.
- Land each wire on the matching letter at the new thermostat.
If the wiring matches no diagram, or you find thick conductors joined with wire nuts (that’s line voltage, not 24V control wiring), stop and get a pro’s eyes on it.
⚠️ Know when to stop
Thermostat wiring is 24 V AC low voltage — safe to handle once you cut power at the breaker (do it anyway; a short can kill the transformer or blow the board fuse). But stop and call an HVAC pro or electrician if you hit any of these:
- Line voltage (120/240 V): thick wires with wire nuts, often on baseboard or wall-heater thermostats. Miswiring these can injure you or start a fire.
- Gas components: anything past the furnace control board — gas valves, ignitors, pilot assemblies.
- Refrigerant lines or sealed-system parts on heat pumps and AC units.
- No idea which system you have: if the wiring doesn’t match any diagram here, photograph it and get a pro’s eyes on it before guessing.
FAQ
Is there a universal thermostat wiring color code?
No. The red-R, white-W, yellow-Y, green-G, blue-C pattern is a widely followed convention, but no standard enforces it, and Honeywell’s own documentation says colors vary by installer. Always match the terminal letter on the old thermostat.
What is the blue wire on a thermostat?
Usually C (common), which gives the thermostat constant 24V power. On Rheem and Ruud heat pumps it can be B, the heating-mode reversing valve. Check which terminal it occupied on the old thermostat; details in can I use the Nest blue wire as a C wire.
What color is the C wire?
Most often blue, sometimes black, occasionally anything else. Verify with a multimeter: R to C reads a constant 24 to 28V AC whether or not the system is running.
Do Honeywell and Nest use different wire colors?
No. Wire colors belong to your house’s cable, not the thermostat brand. Terminal letters are universal across Honeywell, Nest, ecobee, and every other 24V thermostat, which is why matching letters (not colors) always works.
What do I do if my wire colors don’t match any chart?
Go by the terminal letters on the old thermostat, or map each conductor with a multimeter in continuity mode with the power off. The step-by-step recovery procedure is above, and the full replacement walkthrough is in the thermostat wiring guide.
Last reviewed: July 2026
