What is the design capacity characteristic of a Single Duct Constant Volume system?

• A. It can meet the peak load of any space.
• B. It cannot meet peak loads in large spaces.
• C. It always overcools spaces.
• D. It is limited to a fixed number of zones.

Answer: (A)

In a single duct constant-volume system, the central air handler delivers a fixed amount of supply air to a common duct, and zone control is achieved mainly with dampers and zone reheat. Because the total airflow is fixed, you can direct essentially the entire supply to the space with the largest instantaneous load, using that zone’s reheat or cooling to bring its temperature to the desired setpoint. In other words, the design can meet the peak cooling or heating load of any single space by allocating the full supply to that space, assuming the central AHU is sized to provide the required peak flow.

This is the defining characteristic: the system’s capacity is tied to distributing a fixed volume of air, so meeting the peak load of a space is possible by concentrating the available air there. It’s worth noting that simultaneous peaks in multiple spaces aren’t as easily accommodated, which is the trade-off of a single-duct CV arrangement.