Which component is considered part of the primary system in an HVAC setup?

The air handler is considered part of the primary system in an HVAC setup because it plays a crucial role in the movement and management of air throughout the system. The air handler is responsible for circulating conditioned air into the ductwork, which then distributes it throughout the building. It typically contains crucial components such as fans, filters, and heating or cooling elements, making it essential for the overall operation of the HVAC system.

In contrast, while ductwork is necessary for air distribution, it does not actively condition or circulate air; it merely serves as a conduit for the air supplied by the air handler. The thermostat, on the other hand, is a control device that regulates the temperature but is not part of the mechanical system that directly handles air distribution. The expansion valve is part of the refrigeration cycle, controlling refrigerant flow, but it also does not directly manage air circulation. Thus, the air handler is integral to the primary system as it combines several functions that are vital for HVAC operation.