1. Hood exhausts should be manifolded together except for:
- Perchloric/hot acid hoods
- hoods with washdown equipment
- hoods that could deposit highly hazardous residues on the ductwork
- exhaust requiring HEPA filtration or other special air cleaning
- situations where the mixing of exhausted materials may result in a fire, explosion, or chemical reaction hazard in the duct system
Manifolded fume hood exhaust ducts shall be joined inside a fire rated shaft or mechanical room, or outside of the building at the roofline.
CCR, Title 8, Section 5143
NFPA 45
2. Horizontal ducts must slope at least 1 inch per 10 feet downward in direction of airflow to a suitable drain or sump.
ANSI/AIHA Z9.5-1992, 6.1
Liquid pools and residue buildup which can result from condensation may create a hazardous condition if allowed to collect.
3. Ducts exhausting air from fume hoods should be constructed entirely of non- combustible material. Gaskets should be resistant to degradation by the chemicals involved and fire resistant.
NFPA 45, Chapter 6-5.1
4. Automatic fire dampers shall not be used in laboratory hood exhaust systems. Fire detection and alarm systems shall not be interlocked to automatically shut down laboratory hood exhaust fans.
NFPA 45, Chapter 6-10
Fire dampers are not allowed in hood exhaust ducts. Normal or accidental closing of a damper may cause an explosion or impede the exhausting of toxic, flammable, or combustible materials in the event of a fire.
