Wire My ThermostatThermostat wire letters explained
| Letter | Meaning |
|---|---|
| R | 24 V power (hot) |
| RH | Heating transformer power |
| RC | Cooling transformer power |
| C | Common wire (C wire) |
| W | Heat |
| W1 | Heat, stage 1 |
| W2 | Heat stage 2 or aux heat |
| Y | Cooling |
| Y1 | Cooling, stage 1 |
| Y2 | Cooling, stage 2 |
| G | Fan |
| O/B | Heat pump reversing valve |
| O | Reversing valve (energized in cooling) |
| B | Reversing valve OR common: we check |
| AUX | Auxiliary heat (heat pumps) |
| E | Emergency heat |
| W3 | Heat, stage 3 |
| X2 | Aux / 2nd-stage heat (older heat pumps) |
| X | Brand-specific: common or emergency heat |
| K | Honeywell wire saver (fan + cool combined) |
| PEK | ecobee Power Extender Kit wire |
| L | Heat pump service light |
| S | Sensor |
| T | Outdoor anticipator (older units) |
| U | Honeywell universal output |
| HUM | Humidifier |
| DEHUM | Dehumidifier |
Detailed guides
- R Wire on a Thermostat: What It Is and Where It GoesThe R terminal is 24-volt power from your HVAC transformer
- Rh and Rc Thermostat Wires Explained (Dual Transformer)Wires on both Rh and Rc mean separate heating and cooling transformers
- What Is a C Wire? (And What to Do If You Don’t Have One)The C wire (common) gives smart thermostats constant power
- W and W1 Thermostat Wire: Heat (and Aux Heat on Heat Pumps)W or W1 calls for heat on a furnace or boiler
- W2 and AUX Thermostat Wires: Second-Stage and Auxiliary HeatW2 is second-stage heat on a furnace or auxiliary heat on a heat pump
- Y, Y1 and Y2 Thermostat Wires: Cooling and Compressor StagesY turns on your AC or heat pump compressor; Y2 is a second compressor stage
- G Wire on a Thermostat: Fan Control ExplainedThe G terminal runs your furnace or air handler blower fan
- O/B Thermostat Wire: Heat Pump Reversing Valve (O vs B Setting)O/B controls a heat pump’s reversing valve
- B Wire on a Thermostat: Common Wire or Reversing Valve?A B terminal means different things by brand: a heat pump valve on Rheem/Ruud, a common (C) wire on many Trane and Carrier systems
- E Terminal on a Thermostat: Emergency Heat WiringThe E wire runs emergency heat on a heat pump
- X and X2 Thermostat Wires ExplainedX2 is auxiliary heat on older heat pump thermostats; X means common on some brands and emergency heat on others
- K Terminal on a Thermostat (Honeywell Wire Saver)A K wire means a Honeywell wire saver is installed
- L, S and T Thermostat Terminals: Can You Skip Them?L (service light), S (sensor) and T (anticipator) wires usually aren’t needed on a smart thermostat