What are the U wire terminals for?

The Lyric is able to manage a wide range of climate control systems to maximize your comfort, including your heating and cooling systems as well as other indoor air quality (IAQ) systems, such as a whole house humidifier or dehumidifier or ventilation system. The U wire terminals are used to control indoor air quality equipment. Refer to the wiring configuration on your previous thermostat to wire your Lyric properly to your whole house humidifier or dehumidifier or your ventilation system.
In some cases, your IAQ system may be controlled by a separate panel, not by your old thermostat. If so, consult your contractor for advice on connecting that system to your Lyric. For a list of contractors trained to assist in Lyric installation and configuration, go here.
Alternatively, if you have installed the Lyric app on your mobile device, go to the home screen and tap the three vertical dots icon in the upper right hand corner. Tap "Contractor Information" and select your location for a list of local approved contractors.