What is the purpose of demand-controlled ventilation?

• A. To minimize energy use by cycling fans on/off.
• B. To adjust outside air intake based on occupancy or CO2 levels.
• C. To maintain a constant outside air flow irrespective of occupancy.
• D. To maximize outside air penetration in all conditions.

Answer: (B)

Demand-controlled ventilation uses measurements of occupancy or CO2 levels to scale the amount of outdoor air delivered to a space. The idea is to provide enough ventilation to keep indoor air quality acceptable when spaces are crowded, while avoiding wasteful heating, cooling, and fan energy when occupancy is low. If more people are present or CO2 concentrates rise, the system increases outdoor air to dilute contaminants; if occupancy drops or CO2 is low, it reduces outdoor air to save energy. This makes ventilation responsive to actual use rather than fixed at a constant rate.

Cycling fans on/off is a separate energy-saving tactic and does not embody the primary purpose of DCV. Maintaining a constant outdoor air flow regardless of occupancy ignores energy trade-offs and can over-ventilate empty spaces. Maximizing outside air in all conditions wastes energy and can create uncomfortable conditions or IAQ issues when outdoor air is unsuitable.