Fixed Ventilation Systems

Ventilation rates are traditionally based on a maximum assumed occupancy level. For example, 15 cubic feet per minute (cfm) of fresh outside air per person multiplied by the maximum design occupancy. Ventilation air is fixed at this rate to ensure fresh air needs are met at any occupancy level.

Fixed ventilation may be adequate in buildings with no significant changes in occupancy. Over-ventilation can still occur when actual occupancy falls below design levels.

The cost to heat, cool, humidify, and dehumidify excess ventilation air can be significant. For example, it costs roughly $3,500 per year in natural gas to heat 1,000 cfm of fresh outside air, 24 hours a day, 7 days a week. Energy costs are incurred unnecessarily if your building is over-ventilated.

Demand-controlled ventilation may be a better option if occupancy within your building varies.