Different thermostat manufacturers use universal (U, U1, U2) wires to control all kinds of different things.
- HVAC accessories (i.e., humidifiers, dehumidifiers, and ventilators)
- High fan speed in a multi-speed fan system
- Third stage of heating or cooling in a multistage HVAC system
As a result, it's difficult to answer that question without knowing lots of details about your specific system. We strongly suggest a professional installation for these systems because incorrect setup, either due to miswiring or wrong software settings, can cause weird heating, ventilation, and air conditioning (HVAC) behaviour and even damage the HVAC system.
Professional installation services - Google Nest Help
