Odour control and exhaust re-entrainment

Few complaints are as persistent as an unexplained odour that comes and goes. Kitchen smells in the boardroom on certain afternoons, diesel fumes on the third floor whenever the trucks idle, a chemical whiff in the lab office only when the wind blows from the west. In many of these cases, exhaust leaving the building is being drawn straight back in through an outdoor air intake, a window or an open door. This is called re-entrainment, and it is mostly a problem of geometry and airflow around buildings.

This guide explains how exhaust plumes behave, which design practices prevent re-entrainment, and how to investigate odour problems in existing buildings.

How air flows around buildings

Wind striking a building separates at its edges and forms zones of disturbed airflow:

• A recirculation cavity on the roof just downwind of the leading edge, where air swirls back toward the edge
• A wake on the downwind side of the building, where air is turbulent and can flow back toward the leeward wall
• Rooftop structures such as penthouses, screens and parapets, each with their own small recirculation zones

An exhaust plume released inside one of these zones can be trapped and carried to any intake within the same zone, regardless of direction. ASHRAE Handbook, Fundamentals, chapter on airflow around buildings, provides methods for estimating the size of these zones and the dilution of exhaust plumes at intake locations. Nearby taller buildings can also create downwash that brings plumes back to roof level.

Common sources

Design measures to prevent re-entrainment

Separation distances

ASHRAE Standard 62.1 includes a table of minimum separation distances between outdoor air intakes and various sources, ranging from a few metres for lightly contaminated exhaust to roughly 10 m (30 ft) for noxious or dangerous exhaust. Sources such as cooling towers, truck loading areas, garage entrances and combustion vents each have their own values. These are minimums, not guarantees: in strong recirculation zones, even generous distances can fail. NFPA 96 requires kitchen exhaust outlets to be separated from air intakes, and CSA B149.1 sets vent termination clearances for gas appliances. Provincial codes may adopt or modify these requirements. See outdoor air intake placement and design.

Stack height and exit velocity

The most effective way to keep exhaust out of the roof recirculation zone is to release it high enough and fast enough to escape. For laboratory exhaust, ANSI/ASSP Z9.5 recommends stacks discharging at least 3 m (10 ft) above the adjacent roof line and a minimum exit velocity of about 15 m/s (3,000 fpm), along with vertical discharge without rain caps that deflect the plume downward. Similar principles apply to other contaminated exhaust. Stack height and velocity can be traded against each other: a higher effective plume rise reduces the required physical height.

Discharge configuration

• Discharge vertically upward. Rain caps and gooseneck outlets direct exhaust back toward the roof.
• Use drain-type stackheads or stack drains to manage rain without blocking the plume.
• Group exhaust stacks together where possible, since combined plumes rise higher.
• Keep stacks away from the leading edge of the roof and from rooftop screens that create local turbulence.

High-plume and induced-air fans

Induced-flow exhaust fans entrain outdoor air at the fan to increase total discharge volume and momentum, producing a higher plume. They are widely used on laboratory and hospital buildings where tall stacks are undesirable. For manifolded lab systems, see manifolded lab exhaust systems.

Intake location

Place intakes upwind of exhaust relative to prevailing winds where possible, on a different face of the building than exhaust, and away from loading docks, garage ramps and street-level traffic. Roof intakes close to exhaust fans are the most common design error.

Dispersion modelling and wind tunnel studies

For laboratories, hospitals, buildings with generators and buildings near taller neighbours, numerical dispersion modelling or scale wind tunnel testing estimates contaminant concentrations at each intake across a range of wind speeds and directions. Results guide stack heights and exit velocities and can confirm compliance with health-based or odour-based criteria.

Odour control at the source

Sometimes the plume cannot be raised enough, or neighbours are close. Source treatment options include:

• Grease and odour control units for kitchens, using filtration, electrostatic precipitation, activated carbon or UV oxidation, common in mixed-use buildings where kitchen exhaust cannot reach the roof
• Activated carbon or chemical media filters for odorous process or waste room exhaust
• Biofilters and scrubbers for industrial and wastewater sources
• Containment and negative pressure so odours are captured by exhaust rather than escaping into corridors

Odour units need frequent maintenance. Spent carbon or clogged filters quickly return a system to complaint status.

Diagnosing odour complaints

When odours appear in an existing building, a structured investigation usually finds the cause:

1. Log every complaint with time, location, description and weather conditions, including wind direction.
2. Correlate complaints with the operation of candidate sources, such as kitchen hours, generator tests or deliveries.
3. Walk the roof and grade to map intakes, exhausts, flues, vents and doors.
4. Check building and space pressurization; a negative building pulls odours in through doors and cracks. See building pressurization and infiltration control.
5. Verify that exhaust fans are running and moving their design airflow, and that dampers are not stuck.
6. Use tracer smoke or a tracer gas test to confirm a suspected path between an exhaust and an intake.
7. Implement fixes: extend stacks, increase exit velocity, relocate intakes, add source treatment or correct pressurization.

Interior paths are also common: shared return plenums, unsealed shafts, dry floor drain traps and transfer grilles can all carry odours between spaces. Balancing exhaust systems and confirming pressure relationships, part of routine air balancing, resolves many of these.

Key takeaways

• Re-entrainment happens when exhaust is released into a recirculation zone that also contains an intake.
• ASHRAE 62.1, NFPA 96, ANSI/ASSP Z9.5 and CSA B149.1 provide separation and discharge guidance; they are minimums.
• Tall stacks, high exit velocity and vertical discharge without rain caps keep plumes out of roof recirculation zones.
• Dispersion modelling or wind tunnel testing is worthwhile for labs, hospitals and complex sites.
• Odour investigations should correlate complaints with weather and source operation, and check building pressurization.