How-to guide
Thermostat With 6 Wires: What Your System Is and How to Wire It
By Ismail Farooq · Updated 2026-10-01
Six wires behind a thermostat can mean several different systems. Colors won’t tell you which: the letters the wires were connected to will. Take a photo of the old thermostat with the faceplate off before disconnecting anything, then match the letters below.
Common 6-wire combinations
| Letters on the old thermostat | What it usually is | Wiring |
|---|---|---|
| R, C, Y, G, O/B, AUX (or W2) | Heat pump with backup heat and a C wire | Heat pump with C wire |
| R, C, Y, G, B, W2 | Rheem or Ruud heat pump, B is the reversing valve | B valve wire |
| R, Y, G, O, B, W2 | Older Trane, American Standard or Carrier heat pump, B is common | B wire as common |
| R, Y, G, O/B, AUX, E | Heat pump with emergency heat, no C wire | Heat pump, no C wire |
| R, C, W1, W2, Y, G | Two-stage furnace with single-stage AC | Two-stage wiring |
| R, C, W, Y1, Y2, G | Furnace with two-stage AC | Two-stage wiring |
| Rh, Rc, C, W, Y, G | Two transformers (often a boiler plus separate AC), with a C wire | Dual transformer |
| R, C, W, Y, G + HUM | Furnace with AC and a humidifier | 5-wire wiring plus the accessory terminal |
How to tell a heat pump from a furnace
Most 6-wire setups are heat pumps, and the difference matters because the same letters do different jobs. You probably have a heat pump if:
- The outdoor unit runs in winter as well as summer.
- The old thermostat has an “Em Heat” or “Aux” setting or light.
- There is a wire on O, B or O/B.
On a heat pump, Y runs the compressor for heating and cooling, O/B switches between them, and W2/AUX is backup heat. On a furnace, W is the heat. See heat pump thermostat wiring.
Six wires in the cable, five connected
Thermostat cable often has more conductors than the system uses. If six wires come out of the wall but only five were on terminals, the extra one is a spare. That is good news: if your system has no C wire, a spare can often become one. It must be connected to C at the furnace board as well as at the thermostat. See no C wire options.
Two wires that look like the same job
- O and B both used. On older systems that use both, O is the reversing valve and B is common, which goes on C. Putting B on the O/B connector of a new thermostat shorts the transformer.
- AUX and E both used. Many smart thermostats have no E terminal. See heat pump wiring with AUX and E before you connect them, and aux heat vs emergency heat for what each does.
- Rh and Rc both with wires. That means two transformers. Never join them. See Rh and Rc.
Which smart thermostats work with 6 wires?
Most 6-wire systems work with the Nest Learning (4th gen), ecobee Premium and Enhanced, and Honeywell Home T9. The Amazon Smart Thermostat runs one heat and one cool stage and doesn’t support dual-fuel or dual-transformer systems, so it fits fewer of them. The quickest way to be sure is to tick your six letters in the wiring tool: it grades each thermostat for your exact wiring and shows where every wire goes.
After wiring
Set the equipment type (heat pump or conventional), the O/B setting, the number of stages and the aux heat type in the app, then test heat, cool and fan. Our settings checklist covers each one.