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:
  1. Remove your old thermostat’s backplate and check if there’s an unused wire tucked into the wall.
  2. Make sure that wire is connected to the C terminal on your furnace or air handler’s control board.
  3. 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.