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

ThermostatVerdictWhat to know
Nest Learning (4th gen)Works: direct swapDirect swap: every wire has a matching terminal.
ecobee PremiumWorks: direct swapDirect swap: every wire has a matching terminal.
ecobee EnhancedWorks: direct swapDirect swap: every wire has a matching terminal.
Honeywell T9Works: direct swapDirect swap: every wire has a matching terminal.
Amazon Smart ThermostatWorks: direct swapDirect swap: every wire has a matching terminal.

Your wiring is pre-filled below. Change any terminal to match your wall exactly.

1Snap a photo of your old thermostat’s wires (optional)

AI reads the terminal letters and wire colors, then you double-check them. Photos are not saved.

2Tick every terminal that has a wire

Pull the faceplate off your old thermostat. Use the letter printed next to each wire, not its color. Skip short jumper wires between two terminals.

More labels (W3, X, X2, K, PEK, L, S, T, U, HUM, DEHUM)

3A few quick questions

Do you have a heat pump?

A heat pump is an outdoor unit that both heats and cools. Heat pump thermostats usually have an “EM HEAT” or “AUX” setting.

Do any of these apply?

4Your wiring

Your system: 1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
Your B wire is treated as the common (C) wire. That is normal on many Trane, American Standard and Carrier systems. If it lands on a B or O terminal at the equipment, stop and recheck.

Which smart thermostats work with your wires?

Google Nest Learning Thermostat (4th gen)

Works: direct swap
Wires from the wallNest Learning (4th gen)Rred · 24 V powerRc or RhBblue · Common (C wire)CWwhite · Heat, stage 1WYyellow · Cooling, stage 1YGgreen · FanG
WireColorJobConnect toNote
Rred24 V powerRc or Rh
BblueCommon (C wire)C
WwhiteHeat, stage 1W
YyellowCooling, stage 1Y
GgreenFanG

Install steps

  1. Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
  2. Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
  3. Connect each wire to the Nest Learning (4th gen) terminal shown above. Cap unused wires one at a time.
  4. Restore power and finish setup in the app using the settings below.
  5. Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.

App settings to choose

  • Single R wire: the Nest app tells you whether to use Rc or Rh.

What you need

  • Google Nest Learning Thermostat (4th gen)The thermostat itselfCheck price

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Confirm against the official Google Nest install guide. The app also checks your wiring during setup.

Frequently asked questions

Will the Nest Learning (4th gen) work with b wire used as common?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Nest Learning (4th gen) wiring diagram.

Will the ecobee Premium work with b wire used as common?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the ecobee Premium wiring diagram.

Will the ecobee Enhanced work with b wire used as common?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the ecobee Enhanced wiring diagram.

Will the Honeywell T9 work with b wire used as common?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Honeywell T9 wiring diagram.

Will the Amazon Smart Thermostat work with b wire used as common?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Amazon Smart Thermostat wiring diagram.

Other wiring setups