What characteristic defines a constant air-volume HVAC system?

A constant air-volume HVAC system is defined by its ability to deliver conditioned air at a consistent temperature, which is why this characteristic is the most appropriate answer. In these systems, the volume of air being delivered remains constant regardless of the heating or cooling load demands of the space. This means that, while the air temperature may be either heated or cooled, the primary function of the system is to maintain a steady airflow to the conditioned space.

The low-velocity duct system is often utilized in constant air-volume systems because it allows for effective air distribution without excessive noise or energy loss typically associated with high-velocity systems. This is crucial for maintaining occupant comfort in various environments.

The other options, while they describe different aspects of HVAC systems, do not align with the defining characteristic of constant air-volume systems. For example, delivering varying temperature air suggests a different operational approach, typically found in variable air-volume systems. High-velocity duct systems indicate a different design focus, which is more aligned with compact installations and rapid air movement. Lastly, focusing solely on air cooling overlooks the heating capability of constant air-volume systems, which can also provide conditioned air regardless of the temperature changes in a space.