Laboratory HVAC and fume hood exhaust

Laboratory HVAC is first a safety system. It must capture and remove hazardous fumes at their source, dilute any releases in the room, prevent contaminants from spreading to other spaces, and discharge exhaust where it will not return. Comfort and energy efficiency matter too, but they come after safety.

This guide gives an overview of lab HVAC and fume hood exhaust for owners, facility managers and designers. Related articles cover laboratory airflow control systems and manifolded lab exhaust systems in more depth.

Standards that apply

Several standards guide lab ventilation in Canada:

• CSA Z316.5, Fume hoods and associated exhaust systems, covers design, installation and testing of fume hoods and their exhaust.
• ANSI/ASSP Z9.5, Laboratory Ventilation, is widely used and introduces the laboratory ventilation management program.
• NFPA 45, Fire Protection for Laboratories Using Chemicals, addresses fire safety, including hood exhaust requirements.
• ASHRAE Standard 110 sets the method of testing fume hood performance.
• The National Building Code of Canada and provincial building and fire codes govern fire separations, exhaust and ventilation.
• The Canadian Biosafety Standard applies to labs handling biological agents.

Provincial occupational health and safety regulations also apply, and many institutions have their own lab design standards.

Primary and secondary containment

Lab ventilation provides two levels of protection.

1. Primary containment captures contaminants at the source, mainly through fume hoods, biosafety cabinets, snorkels and other local exhaust.
2. Secondary containment relies on room airflow: dilution by air changes and negative pressure so that any escaped contaminants stay in the lab and are exhausted.

Primary containment is the most important. General room air changes cannot make up for a hood that fails to capture.

Fume hood types

Biosafety cabinets are a separate category designed for biological agents; some types exhaust to the building system, others recirculate.

Face velocity and hood location

Typical face velocities for conventional hoods are in the range of 0.4 to 0.5 m/s (80 to 100 fpm), as discussed in fume hood face velocity control and sash sensing. Face velocity alone does not ensure containment. Cross-drafts from supply diffusers, doors and people walking past can pull fumes out of the hood.

Good hood placement:

• Locate hoods away from doors and main walkways.
• Keep supply diffusers away from the hood face, and keep air velocities near the hood low. ANSI/ASSP Z9.5 recommends that cross-draft velocities at the hood face be limited to a fraction of the face velocity.
• Avoid placing hoods opposite each other across narrow aisles.
• Use low-velocity supply diffusers or perforated ceiling panels in hood-intensive labs.

Room airflow

Labs are usually kept negative relative to corridors and non-lab spaces. Supply air is usually 100 percent outdoor air; lab air is not recirculated to other spaces. The supply airflow is set by the largest of hood exhaust demand, minimum ventilation, and cooling load.

Minimum air change rates should be set through a documented hazard assessment, as recommended by ANSI/ASSP Z9.5. Rates vary with lab type and occupancy, and many institutions use reduced rates when labs are unoccupied.

Fume hood exhaust ducts

Materials

Exhaust ducts must resist the chemicals they carry. Common choices include coated galvanized steel, stainless steel and, where permitted, certain plastics. Selection should follow a chemical inventory and consider fire codes. See air valve materials and coatings for corrosive exhaust.

Construction

• Keep exhaust ducts under negative pressure inside the building, with fans located at the end of the system on the roof, so any leaks draw air in rather than pushing contaminated air out.
• Use sealed, welded or gasketed joints suited to the service.
• Slope horizontal runs to drain condensate.
• Route ducts through rated shafts where they penetrate fire separations; NFPA 45 prohibits fire dampers in fume hood exhaust ducts.
• Maintain duct velocities high enough to carry particulates and prevent settling.

Exhaust fans and stacks

Exhaust must leave the building with enough momentum and height to disperse before it can reach intakes, neighbouring buildings or rooftop workers. ANSI/ASSP Z9.5 recommends a stack discharge velocity of at least about 15 m/s (3,000 fpm) and a stack height at least 3 m (10 ft) above the adjacent roof line. Dispersion modelling or wind tunnel studies are used for complex sites or hazardous exhaust.

Individual fans per hood are common in older and smaller buildings. Larger buildings often use manifolded systems with redundant fans and bypass air to maintain stack velocity. Exhaust fans for hoods are usually on emergency power.

Energy in lab buildings

Labs use much more energy per square metre than offices, mainly because of high outdoor air rates. In Canadian climates, heating that outdoor air dominates. Key opportunities include VAV fume hoods, sash management, reduced minimum air changes where safe, exhaust fan optimization and heat recovery that avoids cross-contamination, such as run-around loops.

Testing and certification

Fume hoods should be tested at installation and at regular intervals, commonly annually. Typical tests per ASHRAE 110 and CSA Z316.5:

1. Face velocity measured on a grid across the sash opening.
2. Smoke visualization at the face and inside the hood.
3. Tracer gas containment test, where specified, often at installation.
4. VAV response test for variable volume hoods.
5. Monitor and alarm verification.

Room airflow, pressure direction and exhaust fan operation should be checked at the same time. Annual fume hood certification is standard practice at most Canadian institutions.

Key takeaways

• Lab HVAC is a safety system: capture at the source, dilute in the room, contain with negative pressure and disperse at the stack.
• CSA Z316.5, ANSI/ASSP Z9.5, NFPA 45 and ASHRAE 110 are the key references, alongside building codes.
• Hood placement and low cross-drafts matter as much as face velocity.
• Exhaust ducts stay negative inside the building, resist corrosion, and have no fire dampers.
• Test hoods at installation and regularly, and verify room airflow and stacks at the same time.