What is a function of the thermostat in an HVAC system?

The thermostat plays a crucial role in an HVAC system, primarily by controlling the temperature of the air within a space. It monitors the ambient temperature and communicates with the heating or cooling system to maintain the desired setpoint. When the temperature deviates from the set point, the thermostat activates or deactivates the HVAC system, ensuring comfort for the occupants.

While other options mention important functions that some HVAC system components may perform, they do not pertain directly to the primary role of the thermostat. Regulating refrigerant levels is a responsibility typically associated with the refrigeration cycle and components like expansion valves or compressors. Reducing humidity is usually managed by dehumidifiers or the air conditioning system itself rather than the thermostat. Filtering air contaminants is an important function of air filters within the HVAC system, distinct from the thermostat's control function. Therefore, the thermostat's main function is to control air temperature, making this option the most appropriate choice.