Wiring guide
Heat Pump With a B Wire (Rheem / Ruud Style)
Rheem, Ruud and a few other brands energize the reversing valve in heating mode and label it B. Put it on O/B and choose B in settings, or your heat and cool will be backwards.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Works: direct swap | Direct swap: every wire has a matching terminal. |
| ecobee Premium | Works: direct swap | Direct swap: every wire has a matching terminal. |
| ecobee Enhanced | Works: direct swap | Direct swap: every wire has a matching terminal. |
| Honeywell T9 | Works: direct swap | Direct swap: every wire has a matching terminal. |
| Amazon Smart Thermostat | Works: direct swap | Direct swap: every wire has a matching terminal. |
Where each wire goes, thermostat by thermostat
| Old wire | Nest Learning (4th gen) | ecobee Premium | ecobee Enhanced | Honeywell T9 | Amazon Smart Thermostat |
|---|---|---|---|---|---|
| R (red) | Rc or Rh | Rc | Rc | R | R |
| C (black) | C | C | C | C | C |
| Y (yellow) | Y | Y1 | Y1 | Y | Y |
| G (green) | G | G | G | G | G |
| B (orange) | *OB | O/B | O/B | O/B | O/B |
| W2 (white) | W2 | W1 | W1 | W2 / Aux | W |
Colors are typical, not a rule: always go by the letter the wire was connected to. Click a thermostat for its full diagram, parts and app settings.
How to confirm this is your setup
A heat pump with a wire on B that goes to the reversing valve (on Rheem/Ruud, often orange), and a separate wire on C. The quickest check: if there is also a wire on C, B is probably the valve. If there is no C wire and B is blue, see the B-as-common setup instead.
Mistakes to avoid
- Choosing “O” in the app. With a B-type valve, heating and cooling will be reversed.
- Treating B as a common wire. On this equipment it energizes the valve, so it belongs on O/B.
Our advice
Put the B wire on the new thermostat’s O/B terminal and choose “B” (energized in heat) during setup. W2 here is the backup electric heat and goes to the AUX or W2 terminal, depending on the thermostat.