At thermostat:

• R

• C

• Data A

• Data B

At EIM breakout:

• R – 24V power

• C – Common

• W1 / W2 – Heat stages

• Y1 / Y2 – Cooling stages

• G – Fan

• O/B – Reversing valve

• HUM – Humidifier

• DHUM – Dehumidifier

• VENT – Ventilator

• Rc – Cooling transformer power

• Rh – Heating transformer power

• C – Common

• W / W1 – First-stage heat

• W2 – Second-stage heat

• Y / Y1 – First-stage cooling

• Y2 – Second-stage cooling

• G – Fan relay

• O/B – Reversing valve

• Aux / E – Auxiliary or emergency heat

• S1, S2 – Outdoor sensor inputs

• A – Data +

• B – Data –

• C – 24V common

• D – 24V hot

• S1 – Data / power

• S2 – Data / power

• S3 – Data / power

• Rc – Cooling transformer power

• Rh – Heating transformer power

• C – Common (mandatory for all Ecobee models)

• W1 – First-stage heat

• W2 – Second-stage heat

• W3 – Third-stage heat (on supported models)

• Y1 – First-stage cooling

• Y2 – Second-stage cooling

• G – Fan relay control

• O/B – Reversing valve for heat pump

• Aux / E – Auxiliary or emergency heat

• ACC+ / ACC– – Accessory terminals for humidifier, dehumidifier, or ventilator

• R – 24V power

• C – Common

• W1 – First-stage heat

• W2 – Second-stage heat

• Y1 – First-stage cooling

• Y2 – Second-stage cooling

• G – Fan

• O/B – Reversing valve

• Aux / E – Auxiliary or emergency heat

• Rc – Power from cooling transformer

• Rh – Power from heating transformer

• C – Common (not required but strongly recommended for reliable power)

• W1 – First-stage heat

• W2 – Second-stage heat (when supported)

• Y1 – First-stage cooling

• Y2 – Second-stage cooling (when supported)

• G – Fan relay

• O/B – Reversing valve for heat pump (set in software)

• ★ (Star terminal) – Flexible accessory: humidifier, dehumidifier, emergency heat, fan, or ventilation depending on setup

• 2024 Nest baseplate extras – Some models add dedicated humidifier, dehumidifier, ventilator outputs, breaking them out instead of relying only on the star terminal

• • Rc – 24V power from cooling transformer

  • Rh – 24V power from heating transformer

  • C – Common return path for 24V power

  • W / W1 – First-stage heat

  • W2 – Second-stage heat

  • Y / Y1 – First-stage cooling

  • Y2 – Second-stage cooling

  • G – Fan relay control

  • O/B – Reversing valve (configurable: O = cool energized, B = heat energized)

  • Aux / E – Auxiliary or emergency heat (heat pump systems)

  • L – System monitor input (triggers service indicator light on thermostat)

  • S1, S2 – Outdoor temperature sensor input

  • U1, U2 – Universal terminals for humidifier, dehumidifier, or ventilation

• Rc – Cooling transformer power

• Rh – Heating transformer power

• C – Common (required for smart/WiFi models)

• W1 – First-stage heat

• W2 – Second-stage heat

• Y1 – First-stage cooling

• Y2 – Second-stage cooling

• G – Fan control

• O/B – Reversing valve (configurable for O or B)

• Aux / E – Auxiliary or emergency heat

• HUM – Whole-house humidifier control

• DEHUM (or DHM) – Whole-house dehumidifier control

• L – System monitor / fault indicator input

• S1, S2 – Outdoor sensor connections

• R – 24V power

• C – Common

• W1 – First-stage heat

• W2 – Second-stage heat

• Y1 – First-stage cooling

• Y2 – Second-stage cooling

• G – Fan

• O – Reversing valve (energized in cooling)

• B – Reversing valve (energized in heating)

• E / Aux – Emergency or auxiliary heat

• R – 24V hot

• C – Common

• i+ – Data +

• i– – Data –

• Rc – Cooling transformer power

• Rh – Heating transformer power

• C – Common

• W / W1 – Heat stage 1

• W2 – Heat stage 2

• Y / Y1 – Cooling stage 1

• Y2 – Cooling stage 2

• G – Fan

• O/B – Reversing valve

• E – Emergency heat

• Aux – Auxiliary heat

• R – 24V power

• C – Common (mandatory for Matter integration)

• W – Heat call

• Y – Cooling call

• G – Fan

• O/B – Heat pump reversing valve

• Optional Accessory – Depending on firmware, can support humidifier/dehumidifier via an accessory terminal

• R – 24V power

• C – Common return path

• W1 – First-stage heat

• W2 – Second-stage heat

• Y1 – First-stage cooling

• Y2 – Second-stage cooling

• G – Fan relay

• O/B – Reversing valve (system setup determines mode)

• X2 – Emergency heat (or auxiliary)

• S1, S2 – Outdoor sensor inputs

• Trane “ABCD” terminals – On communicating stats (ComfortLink II, XL850/1050), only four wires: A = Data+, B = Data–, C = 24V common, D = 24V hot. These don’t use standard R/W/Y/G at all.

• B – Used as common in some Trane thermostats (especially older models and heat pump stats). This often confuses techs because on many brands “B” = reversing valve in heating, but in Trane it may be common.

• X or X2 – On older Trane stats, this can also act as common, though it’s sometimes emergency heat. Always check the wiring diagram.

• In these models, if you’re powering a WiFi thermostat, the “B” or “X” terminal is where you’ll grab C.

• Rc / Rh – Power (jumper often provided).

• C – Common (needed for WiFi).

• W / W2 / E – Heat stages & emergency.

• Y / Y2 – Cooling stages.

• G – Fan.

• O/B – Reversing valve.

• L – System monitor (flashes error/service codes).