What function does a thermostat serve in an HVAC system?

A thermostat serves the critical function of signaling the HVAC system to heat or cool the space to maintain a set temperature. It monitors the ambient temperature and compares it to the desired setting. When the actual temperature deviates from the set point, the thermostat sends a signal to the heating or cooling components of the system to activate or deactivate, thereby regulating the indoor climate effectively.

In the context of an HVAC system, controlling air flow direction, indicating system pressure, and measuring humidity levels are other important functions. However, they do not define the primary role of a thermostat, which is to gauge temperature and adjust the heating or cooling system accordingly. This essential function of maintaining comfort through temperature control is why signaling the system to heat or cool is the correct answer.