What is the function of the "R" terminal on the thermostat?

The "R" terminal on a thermostat serves a critical function by providing power from the transformer to the thermostat itself. In residential heating and cooling systems, the "R" terminal is typically connected to the 24V transformer, which supplies low-voltage electrical power to operate the thermostat and, consequently, control the heating or cooling equipment. This power enables the thermostat to communicate with and control the system's components, ensuring that they function correctly according to the desired temperature settings.

When the thermostat is set to call for heating or cooling, it will close the circuit through the "R" terminal, allowing power to flow to other terminals that control various functions such as the compressor, fan, or heating elements. This foundational role of providing power is essential for the overall operation of climate control systems. Understanding the function of the "R" terminal is crucial for anyone working with thermostats in HVAC systems.