Exhaust system design for commercial buildings

Exhaust systems remove air that should not be recirculated: odours from washrooms, moisture from showers, grease from kitchens, vehicle fumes from garages, chemicals from labs and heat from equipment rooms. They also drive building pressure and air movement, so a poorly designed exhaust system can cause problems well beyond the rooms it serves.

This guide covers the main types of commercial exhaust, how exhaust rates are set, fan and discharge considerations, makeup air, and the Canadian code context.

Types of exhaust systems

Setting exhaust rates

Washrooms and general spaces

ASHRAE Standard 62.1 includes minimum exhaust rates for spaces such as public toilets, private toilets, janitor closets and locker rooms. Typical values for public toilets are in the order of 25 L/s (50 cfm) per water closet or urinal, with alternative minimums for continuous versus intermittent operation. Provincial codes and the NBC reference ventilation standards in different ways, so the governing value depends on the jurisdiction. See ASHRAE Standard 62.1 Ventilation Rate Procedure.

Kitchens

Kitchen hood exhaust rates depend on the hood type, the cooking appliances (light, medium, heavy or extra-heavy duty) and hood listing. Listed hoods are tested to UL 710 and ULC equivalents, and the listing sets the minimum airflow. NFPA 96 governs grease duct construction, clearances and fire suppression. Our guide to commercial kitchen ventilation and NFPA 96 covers this in detail.

Garages

Parking garage ventilation in Canada is governed by the NBC and provincial codes, typically with continuous ventilation or CO and NO2 detection that ramps fans. See garage ventilation and CO and NO2 detection.

Laboratories

Lab exhaust is driven by fume hood airflow, minimum air change rates and the need to keep the lab negative to surrounding spaces. Fume hood performance is governed by ANSI/ASHRAE 110 testing, with design guidance in ANSI/ASSP Z9.5 and CSA Z316.5.

Fan types and placement

Locating fans at the end of the system

Exhaust fans are best located at the discharge end, usually on the roof, so that the ductwork inside the building is under negative pressure. Any leakage then flows into the duct, not out of it. This is particularly important for kitchens, labs and any exhaust carrying odours or hazardous contaminants.

Common fan types

• Roof-mounted centrifugal upblast or downblast fans for general and toilet exhaust. Upblast fans are used for kitchen grease exhaust.
• Inline centrifugal or mixed-flow fans where roof fans are impractical, with care to keep positively pressurized duct short and sealed.
• High plume laboratory exhaust fans that entrain outdoor air at the fan and discharge at high velocity to carry contaminants away from the building.
• Utility set fans for larger or higher-pressure systems.

EC motors and variable speed drives allow exhaust fans to modulate with demand, such as reduced toilet exhaust when unoccupied (where code permits) or lab exhaust tracking fume hood use.

Discharge location and re-entrainment

Exhaust that re-enters the building through outdoor air intakes, operable windows or doors defeats the purpose of the system. Re-entrainment is a common source of odour complaints.

Good practice includes:

1. Separate exhaust outlets from outdoor air intakes by the minimum distances in ASHRAE 62.1 Table 5-1 or the applicable code, which increase with the type of exhaust.
2. Discharge vertically upward at sufficient velocity rather than horizontally or downward, where practical.
3. Account for prevailing winds, nearby taller buildings and rooftop features that can create recirculation zones.
4. For labs and other hazardous exhaust, consider a dispersion study using wind tunnel testing or computational methods per ASHRAE Handbook, Fundamentals, airflow around buildings chapter.
5. Avoid rain caps on exhaust stacks that deflect discharge downward; use designs that allow vertical discharge while managing rain.

See odour control and exhaust re-entrainment for more.

Makeup air and building pressure

Every litre of air exhausted must be replaced. If makeup air is not provided by the HVAC system, it will enter through doors, windows and envelope leaks. Consequences include:

• Doors that are hard to open or slam shut.
• Cold drafts and frozen pipes near the envelope in winter.
• Backdrafting of combustion appliances and flues.
• Kitchen hoods that fail to capture smoke.
• Odour migration from one tenant to another.

Commercial buildings are generally designed to be slightly positive relative to outdoors, so total outdoor air supply should exceed total exhaust. Kitchens, labs and washrooms are designed negative relative to adjacent spaces, by supplying less air to them than is exhausted. Large kitchen exhaust often needs dedicated makeup air units, which in Canadian climates must be heated.

Energy recovery from exhaust

General exhaust, toilet exhaust in many cases, and some lab exhaust represent large amounts of conditioned air leaving the building. NECB requires heat recovery from exhaust on many systems, subject to thresholds and exceptions. The type of recovery device must suit the exhaust: wheels may be acceptable for general exhaust, while plate exchangers or run-around loops are often preferred for labs, kitchens and contaminated exhaust to avoid cross-contamination. Kitchen grease exhaust typically needs special consideration because grease fouls heat exchangers.

Ductwork

• Size exhaust ducts with the same care as supply, considering velocity and noise.
• Seal to the appropriate SMACNA class, especially for odorous or hazardous exhaust.
• Use materials suited to the exhaust: galvanized steel for general exhaust, welded steel per NFPA 96 for grease, stainless steel or coated duct for corrosive lab exhaust.
• Provide cleanouts where required, such as grease ducts and dryer exhaust.
• Address fire separations with fire dampers where permitted; grease and some lab ducts may not use fire dampers and require shafts or other protection instead.

Balancing and verification

Exhaust systems should be balanced along with supply systems so that design pressure relationships are achieved. Measurements typically include fan airflow and static pressure, airflow at each grille or hood, and pressure differences across doors to critical spaces. Air balancing confirms these values and identifies issues such as closed dampers or undersized makeup air.

Key takeaways

• Exhaust rates come from ASHRAE 62.1, NFPA 96, lab standards and provincial codes, depending on the space.
• Locate exhaust fans at the discharge end to keep ductwork inside the building negative.
• Discharge upward, away from intakes, and consider dispersion for hazardous exhaust.
• Always provide makeup air and design for intended building pressure relationships.
• Recover energy from exhaust where code requires it, choosing devices suited to the contaminants.