Horizontal air ducts and plenums shall be provided with service openings to facilitate the removal of accumulations of dust and combustible materials. Service openings shall be located at approximately 20-foot (6096 mm) intervals along the air duct and at the base of each vertical riser.
- Removable air outlet or air inlet devices of adequate size shall be permitted in lieu of service openings.
- Service openings shall not be required in supply ducts where the supply air has previously passed through an air filter, air cleaner, or water spray. Such air filters and air cleaners shall be properly maintained and replaced when needed.
- Service openings shall not be required where all of the following conditions exist:
- The occupancy has no process producing combustible material such as dust, lint, or greasy vapors. Such occupancies include banks, office buildings, houses of worship, hotels, and health care facilities (but not kitchens, laundries, and manufacturing portions of such facilities).
- The air inlets are at least 7 feet (2134 mm) above the floor or are protected by corrosion-resistant metal screens of at least 14 mesh (0.07 inches) (1.8 mm) that are installed at the inlets so that they cannot draw papers, refuse, or other combustible solids into the return air duct.
- The minimum design velocity in the return duct for the particular occupancy is 1000 feet/minute (5.080 m/s).