What is a key constraint of a single duct variable volume system?

• A. Must provide ventilation when occupied and required system ventilation under all operating conditions.
• B. Must minimize outdoor air at all times.
• C. Must operate without any fans.
• D. Must only heat, not cool.

Answer: (A)

In a single duct variable volume system, the essential constraint is that the space must receive the required outdoor ventilation whenever it is occupied, and the system must meet the required ventilation rate under all operating conditions. The VAV boxes adjust the amount of supply air to each zone to control temperature, but the outdoor air portion is governed by ventilation standards (like ASHRAE 62.1) and cannot be entirely sacrificed to save energy. So even when the system is at minimum cooling load or running reheat, the building must still receive the mandated ventilation air.

This keeps occupants' air quality in check while still allowing temperature control through varying airflow. The other statements aren’t correct because a single duct VAV system can, and must, provide ventilation, it does rely on fans to move air, and it is capable of both heating and cooling (not restricted to heating only).