When wiring a Cielo Smart Thermostat, you’ll see various letters on the thermostat terminals. Each letter represents a specific function of your HVAC system. This guide explains what they mean and how to connect them correctly.
Before starting, always:
- Turn off the power to your HVAC system from the breaker.
- Use the thermostat compatibility checker to confirm your system works with the Cielo Smart Thermostat.
Common Thermostat Wires and Their Functions
| Color (Terminal) | Function | |
|---|---|---|
| Red (R) |  | Power | 
| Red (RC) |  | Cooling power | 
| Red (RH) |  | Heating power | 
| Yellow (Y Wire) |  | Cooling | 
| Yellow (Y1) |  | Stage 1 cooling. Both heating/cooling in case of a heat pump | 
| Yellow (Y2) |  | Stage 2 cooling | 
| White (W) |  | heating | 
| White (W1) |  | Stage 1 heating | 
| White / Brown (W2) |  | Stage 2 heating | 
| Green (G) |  | Fan | 
| Black/ Blue (C-Wire) |  | Smart thermostat power supply | 
| Orange/ Blue (OB) |  | Reverse valve heat pump | 
| No universal color (X/Aux) |  | Auxiliary heat or stage 2 heating | 
| No universal color (S) |  | Outdoor temperature sensors | 
C – Common Wire
- The C wire (short for Common) provides a continuous flow of 24-volt AC power to your thermostat.
- Without this wire, your smart thermostat cannot stay powered continuously.
- If you don’t see a C wire:- Remove your old thermostat’s backplate and check if there’s an unused wire tucked into the wall.
- Make sure that wire is connected to the C terminal on your furnace or air handler’s control board.
- If not available, you can use the free Common Wire Adapter (CWA) included with your Cielo Smart Thermostat.
 
R Wire
- Supplies 24VAC power from the HVAC system’s transformer to your thermostat.
- If you have only one R wire (that includes R, Rc, and Rh), connect it to the Rc terminal on the Cielo Smart Thermostat.
- Important: Do not use jumper wires with R connections on the Cielo Smart Thermostat.
Rh – Heating Power Wire
- In systems without a separate Rc wire, Rh serves as the power source for both heating and cooling.
- In dual-transformer systems, Rh provides power for the heating transformer, while Rc provides power to the cooling transformer.
- If only one Rh wire is present, it should be connected to Rc terminal on Cielo Smart Thermostat.
Rc – Cooling Power Wire
- In single-transformer systems, Rc provides power for both heating and cooling (when no Rh wire is present).
- In dual-transformer setups, Rc is dedicated to cooling power, separate from Rh for heating.
W / W1 and W2 – Heating Stages
- W/W1: Controls the first stage of heating in conventional systems.
- W2: Controls the second heating stage, used in multi-stage systems for faster or more efficient heating.
- Heat pump systems:- Connect AUX/AUX1 to W1 for auxiliary heat.
- Connect AUX2 to W2 for second-stage auxiliary heat.
 
Y / Y1 and Y2 – Cooling Stages
- Y/Y1: Controls the first stage of cooling in standard air conditioning systems.
- Y2: Controls the second stage of cooling for quicker cooling and efficiency.
- In heat pump systems:- Y1 controls the compressor, which both heats and cools the home.
- Y2 controls the second stage of the compressor for better performance.
 
G or G1 – Fan Wire
- Controls your HVAC system fan.
- Turning the fan on can circulate air without heating or cooling.
O/B – Reversing Valve
- Used in heat pump systems to switch the flow of refrigerant between heating and cooling modes.
- Some systems energize the reversing valve in cooling mode (O), others in heating mode (B). The Cielo Smart Thermostat can be configured to match your system.
E – Emergency Heat
- Found in some heat pump thermostats.
- Activates your backup heating system directly, bypassing the heat pump.
- Only use during extreme cold when the heat pump cannot keep up.
- Connect the E wire to W1 on the Cielo Smart Thermostat.
AUX / AUX1 and AUX2 – Auxiliary Heat
- Used in heat pump systems with extra heating elements.
- AUX1 (or W1) connects to W1 on the Cielo Smart Thermostat.
- AUX2 connects to W2 for second-stage auxiliary heat.
Less Common Thermostat Wires
| Wire Label | Function | Needed for Cielo? | 
| S, S1, S2 | Outdoor temperature sensors | No – Cielo gets weather data via Wi-Fi | 
| L | Indicator light on old thermostats | No – Cielo has built-in display | 
| G2, G3, GL, GM, GH | Multi-speed fan control | Contact Cielo Support | 
| O and B (separate wires) | Separate reversing valve control | Contact Cielo Support | 
| W3 | Third-stage heating | Not supported | 
| Y3 | Third-stage cooling | Not supported | 
| H / HUM / HUM1 / HUM2 | Whole-home humidifier | Optional | 
| D / DH / DHUM1 / DHUM2 / DEHUM2 | Whole-home dehumidifier | Optional | 
| V, VR, Vg, V+ | Variable control wires | Contact Cielo Support | 
Incompatible Thermostat Wires
The following wires are not primarily used by HVAC system is not compatible with the thermostat.
| Wire Label | Description | 
| 1, 2, 3, 4 | Proprietary system wiring | 
| A, B, C, D | Proprietary system wiring | 
| Water or H₂O | Water detection wires – not supported | 
| L1, L2, 110V, 120V, 240V | High-voltage wiring – dangerous and incompatible | 
Any unused wires can be safely capped with a wire nut, wrapped, and tucked back into the wall to avoid short circuits.
