How-to guide
Heat Pump Thermostat Wiring: O/B, AUX and E Made Simple
By Ismail Farooq · Updated 2026-10-29
A heat pump thermostat has a few jobs a furnace thermostat doesn’t:
- Y runs the compressor in both heating and cooling.
- O/B flips the reversing valve. O (energized in cooling) is used by most brands; B (energized in heating) by Rheem and Ruud. More on O vs B.
- AUX / W2 / W1 runs backup electric heat strips when the heat pump can’t keep up.
- E runs emergency heat only (compressor off). Often tied to AUX at the air handler. E wire details.
Dual fuel
If the backup heat is a gas or oil furnace instead of electric strips, it’s a dual-fuel system. Nest, ecobee and Honeywell T9 support dual fuel; Amazon says its Smart Thermostat does not.
After wiring
In the app, choose heat pump, set O/B correctly, and set aux heat type. Then test: cooling should blow cold within a few minutes; heating should blow warm without the aux indicator on (in mild weather).
Typical heat pump wire sets
| Wires on the old thermostat | What it usually is |
|---|---|
| R, C, Y, G, O/B, AUX | Single-stage heat pump with electric aux heat and a C wire (wiring) |
| R, C, Y, G, O, AUX, E | As above, plus a separate emergency heat wire (wiring) |
| R, Y, G, O/B, AUX | Heat pump without a C wire (wiring) |
| R, C, Y, G, B, W2 | Rheem or Ruud heat pump with a B valve wire (wiring) |
| R, C, Y1, Y2, G, O/B, AUX | Two-stage heat pump with aux heat |
Common mistakes
- Setting it up as a furnace. If the thermostat is set to conventional heating, it calls the W (aux) wire for heat and never runs the heat pump in heating mode. Always choose heat pump in the equipment settings.
- Treating a common B wire as the valve. If the old thermostat had wires on both O and B, B is common and goes on C.
- Leaving the fan to the furnace. Heat pumps and electric air handlers need the thermostat to run the fan (G) during heating. Choose electric or heat pump heat, not gas, when the thermostat asks.
- Aux heat allowed at mild temperatures. If the aux indicator comes on often when it isn’t cold, check the aux heat settings: see aux heat running too much.
For a longer explanation of defrost, dual fuel and balance points, read our heat pump and dual-fuel wiring guide.