Wiring guide
B Wire as Common (Trane / American Standard Style)
On many Trane, American Standard and Carrier furnaces, the B terminal on the old thermostat is actually the common wire. Put it on O/B and you can blow a fuse or damage the thermostat.
Detected system: 1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
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 |
| B (blue) | C | C | C | C | C |
| W (white) | W | W1 | W1 | W | W |
| Y (yellow) | Y | Y1 | Y1 | Y | Y |
| G (green) | G | G | G | G | G |
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 furnace (no heat pump) with a wire on B and no wire on C. On many Trane, American Standard and some Carrier systems, B on the old thermostat is the common side of the transformer. At the furnace board, that wire lands on C, COM or B/C.
Mistakes to avoid
- Putting the B wire on O/B because the letters match. On this equipment it shorts the transformer, which can blow the furnace fuse.
- Assuming every B wire is a common. On Rheem/Ruud heat pumps B is the reversing valve. Always check where the wire lands at the equipment.
Our advice
Once you have confirmed that the B wire ends on C or COM at the furnace, connect it to the new thermostat’s C terminal. You now have a C wire and don’t need an adapter.