Honeywell Home T9 Smart Thermostat wiring for b wire used as common

Good news: the Honeywell Home T9 Smart Thermostat is a direct swap for this setup. Every wire has a matching terminal.

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.

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?

Honeywell Home T9 Smart Thermostat

Works: direct swap
Wires from the wallHoneywell T9Rred · 24 V powerRBblue · Common (C wire)CWwhite · Heat, stage 1WYyellow · Cooling, stage 1YGgreen · FanG
WireColorJobConnect toNote
Rred24 V powerR
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 Honeywell T9 terminal shown above. Cap unused wires one at a time.
  4. Restore power and finish setup in the app.
  5. Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.

What you need

  • Honeywell Home T9 Smart ThermostatThe thermostat itselfCheck price

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Confirm against the official Honeywell Home install guide. The app also checks your wiring during setup.

Frequently asked questions

Where does each wire go on the Honeywell T9?

R → R; B → C; W → W; Y → Y; G → G.

Do I need a C wire for the Honeywell T9?

You already have a C wire, so no adapter is needed.

Other thermostats for this setup

Honeywell T9: other setups