Five wires at your thermostat is the setup every smart thermostat manufacturer designs for first: R, C, W, Y, G. Power, common, heat, cool, and fan, all present. There’s no C-wire workaround to research and no power-stealing quirks to troubleshoot. If you’re about to buy a Nest, ecobee, or any other smart model, this is the wire count that makes the install boring in the best way.
This page covers the R/C/W/Y/G hookup and color table. For every other wire count, see the full thermostat wiring guide.
TL;DR
| Wire (typical color) | Terminal | What it does |
|---|---|---|
| Red | R | 24V AC power from the transformer |
| Blue (or black) | C | Common — completes the circuit, powers the thermostat itself |
| White | W | Heat call (R connected to W fires the furnace) |
| Yellow | Y | Cooling call (R connected to Y runs the compressor) |
| Green | G | Fan relay (R connected to G runs the blower) |
Match the terminal letters, not the wire colors. Photograph the old thermostat before disconnecting anything, and label each wire by the letter it was on.
5-wire color code
| 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. |
The table is the convention, and it usually holds on a factory-standard 5-wire run: red to R, blue to C, white to W, yellow to Y, green to G. But there’s no enforced rule, and a previous owner’s DIY rewire can put anything on any color. Go by the terminal letter printed on the old thermostat’s sub-base or, if the old thermostat is gone, by the labeled screw terminals at the furnace control board.
How to wire a 5-wire thermostat, step by step
Safety first: turn off power at the breaker or the furnace switch, and confirm the thermostat display is dark before pulling it off the wall.
- Kill the power. Breaker or the furnace switch. Confirm the display is dead.
- Photograph the wiring. One wide shot of the sub-base, one close enough to read every terminal letter.
- Label each wire by letter: R, C, W, Y, G. Letters, not colors.
- Disconnect and secure the cable so it can’t slide back into the wall.
- Mount the new base and land the wires: each wire to the terminal with the same letter. Seat firmly and tug-test each one.
- Set the installer menu: system type (gas furnace, AC), fan type, and cycling rate.
- Restore power and test all three functions: heat, cool, and fan, each for at least a full minute.
Because C is already present, this is the one wire count where you can skip the entire C-wire troubleshooting conversation. No power-stealing symptoms, no Power Extender Kit, no add-a-wire adapter.
5 wires on a heat pump: R, C, Y, G, O/B
Not every 5-wire system is furnace-style. On some heat pumps, the fifth wire is O/B (the reversing valve) instead of W, giving you R, C, Y, G, O/B rather than R, C, W, Y, G. There’s no separate heat-call wire because a basic heat pump’s heat call runs through Y plus the reversing valve position:
- R: 24V power.
- C: common, constant power.
- Y: compressor, for both heating and cooling calls.
- G: fan.
- O/B: reversing valve direction. O energizes in cooling on most brands (Carrier, Trane, Goodman, Lennox); B energizes in heating on Rheem and Ruud. Set O or B in the installer menu, never by moving wires.
Confirm which layout you have at the old sub-base before assuming furnace-style wiring. Full heat-pump walkthrough, including aux and emergency heat: heat pump thermostat wiring.
Troubleshooting
| Symptom | Most likely cause | Check |
|---|---|---|
| Heat works, no AC | Y wire loose, or wrong system type set | Tug-test Y at both ends; confirm system type in the installer menu |
| AC works, no heat | W wire loose (or, on a heat pump, O/B set backwards) | W terminal screws, or the O/B setting if this is a heat pump |
| Fan won’t stop running | G shorted to R, or fan set to ON | Thermostat fan setting first; then inspect G insulation where the cable is stapled or kinked |
| Blank display after install | Blown control-board fuse from a short during install | The blade fuse on the furnace board; then measure across R and C (should read ~24-28V AC with the stat off the wall) |
FAQ
What is the color code for 5-wire thermostat wiring?
The convention is red to R, blue to C, white to W, yellow to Y, green to G. It’s a convention, not a rule: confirm against the terminal letters on the old thermostat or at the furnace control board.
How do I wire a 5-wire Honeywell thermostat?
Same letters, same landings: R, C, W, Y, G onto the matching Honeywell terminals. If the backplate shows separate Rc and Rh, land the red wire on R (or Rh) and leave the jumper or slider in the single-transformer position for a standard 5-wire home.
Do I need to do anything special because I have a C wire?
No — that’s the point. Land C on C and every smart thermostat gets constant power from day one. No Power Extender Kit, no power-stealing workarounds, no add-a-wire adapter.
Is 5-wire thermostat cable different from 4-wire cable?
Same 18-gauge wire, one more conductor. If you’re ever replacing the cable, pull 18/8 instead of 18/5 so a future heat-pump or two-stage upgrade doesn’t require re-fishing wire: see the wire gauge guide.
My 5 wires don’t match R/C/W/Y/G. What do I have?
Likely a heat pump running R, C, Y, G, O/B instead. Confirm at the old sub-base, then see heat pump thermostat wiring for the full O/B walkthrough.
⚠️ 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.
Last reviewed: July 2026. Wiring conventions verified against Honeywell Home, Google Nest, and ecobee support documentation; always confirm against your equipment’s manual.
