Ventilation

Every occupied space shall be ventilated by natural or mechanical means. The code treats ventilation as a minimum, continuing condition of occupancy — not an optional design feature.

• Ventilation is required to dilute and remove contaminants that are harmful to occupants even when no odor is perceived.
• Outdoor air intakes must be located to avoid drawing in contaminants from nearby sources such as exhaust outlets, loading docks, or other pollution sources.
• The separation between exhaust outlets and outdoor air intakes prevents re-entrainment of stale or contaminated air back into the ventilation system.

Natural ventilation through openings is permitted where the code allows it, but the openings must be readily operable by the occupants and sized and distributed to serve the occupied space.

• Openings that cannot be operated by occupants do not satisfy the natural ventilation requirement.
• Natural ventilation is not considered reliable as a sole method in all occupancies — certain uses require mechanical ventilation.
• When outdoor conditions are unfavorable, the effectiveness of natural ventilation is limited; mechanical systems provide a dependable alternative.

Mechanical ventilation systems must deliver outdoor air to occupied spaces at rates sufficient to protect occupant health. Rates are based on both occupant load and floor area in many space types.

• Systems must operate continuously during periods of occupancy — ventilation may not be shut off while spaces are occupied.
• Supply airflow and outdoor airflow are distinct: supply air may include recirculated air, but the outdoor air fraction must meet code minimums.
• Short-circuiting — where supply air passes directly to exhaust without reaching the occupied zone — does not satisfy the ventilation requirement.
• Outdoor air intakes must be located and maintained so the air delivered is clean and uncontaminated.
• Balanced supply and exhaust airflow prevents excessive negative or positive pressure that could impair system performance.

Exhaust systems must discharge air to the outdoors in accordance with Chapter 5. Exhaust from mechanical ventilation systems shall be discharged not less than 20 feet from a property line or openings into buildings.

• Exhaust air must not be discharged into plenums used for environmental air or into other interior spaces.
• Contaminants must be captured as close as practicable to their source to prevent migration into occupied areas.
• Makeup air must be accounted for when designing exhaust systems to prevent excessive negative pressure.
• Exhaust systems serving contaminant-producing spaces must operate whenever those activities occur.