Ecobee – What are the U or * (Star) Connectors on My Thermostat, and Where Do I Connect These Wires on an ecobee Thermostat?

Category: Thermostats > Compatibility

Answer:

Some thermostat manufacturers use the * (Star), U, U1, or U2 terminals for specialized HVAC functions.

These terminals may control:

• Humidifiers
• Dehumidifiers
• Ventilators
• High-speed fan operation
• Third-stage heating
• Third-stage cooling

Before installing an ecobee thermostat, you must determine what function the wire controls.

Google Nest and Wyze * (Star) Terminal

The * (Star) terminal on Nest and Wyze thermostats may control:

• HVAC accessories
• High-speed fan operation
• Third-stage heating
• Third-stage cooling

My * (Star) Wire Controls a Humidifier or Dehumidifier

During installation:

1. Power off the HVAC system.
2. Connect the wire to ACC+ on the ecobee thermostat.

Good to Know

• ecobee3 lite does not support HVAC accessories.

My * (Star) Wire Controls High-Speed Fan Operation

The wire is used for multi-speed fan control.

Refer to the multi-speed fan installation requirements for proper wiring and compatibility.

My * (Star) Wire Controls Third-Stage Heating or Cooling

ecobee thermostats do not directly support:

• W3 (third-stage heat)
• Y3 (third-stage cool)

Refer to the ecobee multistage HVAC compatibility guidelines for available installation options.

U, U1, and U2 Terminals

U, U1, and U2 terminals may control:

• Humidifiers
• Dehumidifiers
• Ventilators
• Third-stage heating
• Third-stage cooling

Determine the function of the wire before installation.

My U, U1, or U2 Wire Controls a Humidifier or Dehumidifier

Single U Wire

1. Power off the HVAC system.
2. Connect the wire to ACC+.

Two U Wires

1. Power off the HVAC system.
2. Connect one wire to ACC+.
3. Connect the second wire to ACC−.

Good to Know

• Smart Thermostat Essential does not support HVAC accessories.
• ecobee3 lite does not support HVAC accessories.

My U, U1, or U2 Wire Controls Third-Stage Heating or Cooling

ecobee thermostats do not directly support:

• Third-stage heating
• Third-stage cooling

Refer to the ecobee multistage system installation guidelines for supported configuration options.

Good to Know

• U and * terminals do not always perform the same function between thermostat brands.
• Always identify the purpose of the wire before connecting it to an ecobee thermostat.
• Connecting a wire to the wrong terminal may cause HVAC equipment to operate incorrectly.
• If the wire function is unknown, consult the HVAC manufacturer or thermostat manufacturer before installation.