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. The control side is 24V AC, but a short can blow the control-board fuse or damage the transformer.
Six wires at the thermostat almost always means the standard five (R, C, W, Y, G) plus one extra terminal, and which extra terminal you have tells you what kind of system you’re wiring. The two common patterns:
- Two-stage heat, single-stage cooling: R, C, W, W2, Y, G, adding a second white wire for second-stage furnace heat.
- Heat pump with automatic backup heat: R, C, Y, G, O/B, and AUX (or W2), with no plain W terminal at all, because a heat pump’s primary heat runs through Y and the reversing valve.
This page covers both, plus the two-stage-cooling variant (R, C, W, Y1, Y2, G). For every other terminal and wire count, start with the full thermostat wiring guide.
TL;DR
| Your wires | Likely system | Extra terminal | Full guide |
|---|---|---|---|
| R, C, W, W2, Y, G | Two-stage furnace, single-stage AC | W2 (second heat stage) | This page |
| R, C, W, Y1, Y2, G | Single-stage furnace, two-stage AC | Y2 (second cool stage) | See below |
| R, C, Y, G, O/B, AUX | Heat pump with backup heat | O/B (reversing valve) | Heat pump thermostat wiring |
Match the terminal letters, not the wire colors. Photograph the old thermostat before disconnecting anything, and label each wire with the letter it was on.
6-wire color code: R, C, W, W2, Y, G
| 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. |
On this six-terminal pattern:
- R: 24V AC power from the transformer.
- C: common. Completes the circuit so a smart thermostat has constant power.
- W: first-stage heat call.
- W2: second-stage heat call. This is the wire the standard 5-wire pattern doesn’t have. On a two-stage furnace, the thermostat brings on W first, and if the space doesn’t recover fast enough, adds W2 for higher-capacity heat.
- Y: cooling call to the compressor contactor.
- G: fan relay.
As always, colors are a convention the installer may not have followed. Verify by terminal letter on the old thermostat, or trace each conductor to the labeled screw on the furnace control board.
Step-by-step: wiring the two-stage-heat pattern
- Kill the power. Breaker or the switch on the furnace. 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, not color: R, C, W, W2, Y, G.
- Disconnect and secure the cable so it can’t slide back into the wall.
- Mount the new base and land each wire on its matching letter. W2 is the one installers most often skip or mislabel on a swap — tug-test it specifically.
- Set the installer menu to two-stage heat (sometimes called “heat stages: 2” or “W2 enabled”). If you leave the thermostat on single-stage, the W2 wire does nothing and you lose the second-stage capacity you’re paying for in the furnace.
- Restore power and run a full heat cycle. On a cold day, let the system run long enough to see it step up to second stage; on a mild day, force it in the installer test mode if your thermostat has one.
The other 6-wire pattern: two-stage cooling
If your system is a single-stage furnace paired with a two-stage air conditioner instead, the sixth wire is Y2, not W2: R, C, W, Y1, Y2, G. Y1 runs the compressor’s first stage, Y2 adds the second. The installer-menu step is the same idea in reverse: set cooling stages to 2, not heat stages.
Some two-stage systems (a two-stage furnace and a two-stage AC together) need eight wires, not six, because they use W2 and Y2 at once. If you only have six terminals and your system is genuinely two-stage on both sides, check for a folded-back spare conductor in the wall before assuming you need to pull new cable. Many “6-wire” runs are actually 8-conductor cable with two unused wires tucked behind the plate.
The heat-pump version: no W, but O/B and AUX
Not every six-wire job is a two-stage furnace. A heat pump with automatic backup heat also lands on six wires, but the letters are different because a heat pump has no separate primary-heat terminal; heat comes from Y (the compressor) running in reverse, set by O/B:
- R: 24V power (sometimes shown as Rc on a single-transformer heat-pump system).
- C: common.
- Y: compressor, used for both heating and cooling on a heat pump.
- G: fan.
- O/B: the reversing valve. O energizes it in cooling on most brands; B energizes it in heating on Rheem and Ruud. You set which one in the installer menu; you don’t move wires to change it.
- AUX (or W2): automatic backup heat, usually electric strips, that stages in when the compressor alone can’t keep up in cold weather.
If your old thermostat has no W terminal in use at all and instead shows O, B, or AUX, you have a heat pump, not a furnace. The full walkthrough, including the O-versus-B setting and why the aux wire should stage in automatically instead of overriding the compressor, is in heat pump thermostat wiring. Honeywell equipment follows the same terminal letters with a few brand-specific jumpers and sliders; see Honeywell thermostat wiring for those.
How to tell which 6-wire system you have
Before you land a single wire, confirm which pattern you’re looking at:
- Check the old thermostat’s terminal labels first. If you see W and W2 with no O or B anywhere, it’s two-stage heat. If you see O or B with no plain W, it’s a heat pump.
- Look outside. A heat pump has an outdoor unit that runs in both summer and winter. A furnace-only system’s outdoor unit (if any, for the AC) only runs in summer.
- Check the furnace or air handler’s control board. The board’s terminal strip is labeled the same way, and if the old thermostat is gone, that’s where you trace each conductor.
- When in doubt, don’t guess and reverse-engineer by trial and error. Landing a heat-pump O/B wire on a furnace’s W2 terminal (or the reverse) won’t just fail quietly. On a heat pump it can run the system backwards, blowing cold air when you call for heat.
Troubleshooting 6-wire problems
| Symptom | Most likely cause | Check |
|---|---|---|
| First-stage heat works, never steps up on cold days | W2 loose, or heat stages not set to 2 in the installer menu | Tug-test W2; confirm the installer setting |
| Heat pump blows cold air when set to heat | O/B set to the wrong logic for your brand | Installer menu O/B setting; see heat pump thermostat wiring |
| Second-stage cooling never kicks in | Y2 loose, or cooling stages not set to 2 | Tug-test Y2; confirm the installer setting |
| Backup heat runs constantly, even in mild weather | AUX wired to W2 without staging logic set, or thermostat set to emergency heat by mistake | Confirm EM HEAT is off; check aux/backup-heat lockout settings |
| Smart thermostat reboots or drops Wi-Fi | Weak or missing common power despite a landed C wire | Confirm C reads a steady ~24-28V AC against R; see C-wire installation options |
FAQ
What does a 6-wire thermostat wiring diagram show?
Most commonly R, C, W, W2, Y, G for a two-stage furnace with single-stage cooling. The other common six-wire pattern is a heat pump with backup heat: R, C, Y, G, O/B, and AUX. Check your old thermostat’s terminal labels to know which one you have.
What is the sixth wire on a 6-wire thermostat?
On a two-stage furnace, it’s W2, the second-stage heat call. On a heat pump, there’s no W2 in the usual sense — the sixth wire is either O/B (reversing valve) or AUX (backup heat), and the wire count works out to six because there’s no separate W terminal at all.
Does a 6-wire thermostat always mean a heat pump?
No. Two-stage furnace and AC systems commonly use six wires too, with W2 or Y2 as the extra terminal instead of O/B. Look for O or B on the old thermostat: if it’s there, you have a heat pump; if it’s not, you likely have a two-stage furnace or AC.
Can I wire a 6-wire heat pump the same way as a 6-wire furnace?
No. The terminal letters are different because the systems work differently. A heat pump has no W-only heat call; heat runs through Y and the O/B reversing valve. Wiring heat-pump conductors onto furnace-pattern terminals (or the reverse) can run the system in the wrong direction. See heat pump thermostat wiring for the heat-pump-specific hookup.
Is Honeywell 6-wire thermostat wiring different from other brands?
The terminal letters are universal. What differs is hardware: some Honeywell retail models ship with a jumper between Rc and Rh that you leave in place on a single-transformer system, and the T10 Pro uses a slider tab instead of a jumper. See Honeywell thermostat wiring for the brand-specific details.
⚠️ 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, ecobee, and HVACR School documentation; always confirm against your equipment’s own manual.
