Smoke control

Smoke control is the deliberate limitation and direction of smoke movement during a fire. It can use physical barriers, natural ventilation, mechanical extraction, pressurisation or a combination of these measures. The objective is to protect particular spaces or routes for a required period, rather than to remove every trace of smoke from a building.

Smoke control works with the processes that transport combustion products. Hot gases rise, ceiling jets spread sideways, and pressure differences drive flow through openings. Fans, vents and barriers can change those flows, but outdoor conditions, building leakage and other fire systems also influence the result.

The design must therefore state where smoke is intended to go, which spaces must remain usable and what conditions could defeat the system. Detection, make-up air, equipment operation and maintenance belong to the assessment. A fan or vent installed without those supporting functions does not establish smoke control.

Objectives and protected spaces

The IFEG describes the general objective as removing heat and minimising smoke concentration in designated areas. This focuses attention on selected locations rather than an undefined whole-building outcome. The relevant locations might include escape routes, occupied zones or areas needed for firefighting.

Cooper identifies three objectives for designed smoke and heat venting. These are supporting escape, helping firefighters enter and see the fire, and reducing smoke and heat damage to property. A system may address more than one objective, but the corresponding conditions and duration must be made explicit.

A smoke-control zone provides a defined boundary for that objective. It can be separated from adjacent zones by partitions, floors, doors and dampers. NUREG-1805 notes that a smoke compartment is enclosed on all sides, including the top and bottom.

Protection of a boundary is not the same as maintaining clear air throughout the fire room. Pressurisation can restrict smoke entering a stair while leaving severe conditions in the fire compartment. The location of people and the intended place of safety determine whether that outcome is useful.

Extraction has a different spatial aim. It removes smoke from the hot layer to slow descent or reduce concentration in the protected volume. Its effectiveness depends on the incoming smoke flow and replacement air, so a rated fan capacity alone cannot specify the resulting layer height.

Smoke movement as the design basis

Smoke movement begins with the fire plume and its entrainment of surrounding air. The rising products carry smoke into an upper layer, after which ceiling flows and openings distribute it. The amount entering a smoke-control system therefore depends on fire size, plume geometry and air supply.

Pressure differences govern horizontal flow through openings. They can arise from hot gases, wind, building services or differences between indoor and outdoor temperatures. Smoke-control pressures act within that combined field rather than replacing all other influences.

The fire scenario must include the fuel and the geometry that shapes entrainment. IFEG notes that walls and corners restrict air entrainment into a plume. A model developed for an unrestricted fire may consequently misrepresent smoke production near such boundaries.

Layer behaviour provides another design condition. Smoke-layer height varies with time, fire size and exhaust rate. A calculated clear height belongs to those inputs and cannot be treated as a fixed property of a ventilation unit.

This is why smoke-control analysis connects source, transport and protected space. Assessing only the exhaust outlet would omit the amount of smoke arriving and the air paths supplying it. Assessing only the fire would omit the barriers and flows that determine exposure elsewhere.

Natural and mechanical ventilation

Smoke and heat exhaust ventilation (SHEV) can remove hot gases through openings near the ceiling or roof. Cooper distinguishes natural buoyancy-driven removal, fan-driven removal and combined arrangements. The system must also introduce relatively cool, uncontaminated air at a lower level.

Natural ventilation uses the buoyancy of the hot layer and the pressure conditions across vents. It does not require a fan to create every part of the flow, but it still depends on effective vent area, layer conditions and replacement air. Wind and external temperatures can alter those conditions.

Mechanical extraction uses fans to remove smoke from the upper part of a space. Australian ABCB guidance describes extraction at or near the roof or ceiling, with make-up air at low level. That guidance concerns its national context and does not provide a universal design arrangement.

Make-up air replaces gases removed by the exhaust or vent. Insufficient replacement air can restrict the intended flow and change pressure conditions. It should be treated as part of the smoke-control system rather than as an unrelated opening.

Airflow paths should not short-circuit the intended smoke movement. Supply at low level and exhaust near the layer serve different functions in the cited venting approach. Their locations and operation must support a coherent flow from clean inlet to smoky outlet.

Pressure differences and containment

Stair pressurisation supplies air to protect an escape stair from smoke infiltration. It creates a pressure difference between the stair and adjoining spaces. Open doors change the airflow and pressure, so the design must consider the relevant door conditions.

Zone pressurisation uses different pressures between a fire zone and adjacent zones. Supply to non-fire zones and extraction from the fire zone can both contribute. The intended direction is from protected spaces towards less protected spaces through leakage paths.

Excessive pressure is not a universal improvement. ABCB guidance explains that relief may be needed to avoid excessive door-opening forces or interference with automatic closing. The system must protect against smoke while preserving the operation of the escape route.

A Smoke barrier restricts smoke passage through a continuous construction or closure. NUREG-1805 notes that such a barrier may or may not also have fire resistance. Smoke containment and fire resistance are related building functions but are not interchangeable labels.

Dampers can complete a smoke boundary within ventilation ducts. Shutdown of air handling can limit recirculation and smoke transfer between compartments. A closed fan alone may not seal an open duct, so the damper arrangement and its control still matter.

Reservoirs, boundaries and special volumes

A Smoke reservoir is a volume in which smoke can collect above the space being protected. Its boundaries help define where the layer is retained and from which region extraction occurs. Reservoir behaviour must be analysed together with incoming plume flow and replacement air.

Large open spaces differ from subdivided office floors. Cooper describes warehouse spaces that may be divided by ceiling screens and atrium-like spaces connected to adjacent rooms. These geometries affect smoke accumulation and the paths between the fire and occupied areas.

Atrium smoke control must account for the connected volume and the locations requiring protection. Smoke can collect above an atrium fire or arrive from an adjoining space. The relevant plume and layer model therefore depends on the actual fire location.

Tunnel ventilation addresses a different geometry and set of flow paths. Long, confined spaces can direct smoke along the tunnel rather than allowing a broad upper reservoir to form. The source documents support this distinction in geometry but do not provide a complete tunnel design method here.

System interaction and reliability

Smoke control often relies on detection to select the correct fire mode. Zone systems may need the alarm location as well as confirmation that a fire exists. An incorrect zone signal could produce pressure or airflow patterns that do not match the fire location.

Sprinkler operation can reduce heat and buoyancy while disturbing the smoke layer. NIST describes cooling and mixing in its discussion of building smoke management. This interaction can change the conditions assumed for smoke transport and heat-operated vents.

Cooper describes delayed activation of later sprinklers or vents when earlier sprays cool their elements. Such skipping makes the sequence less predictable. Interactions should be evaluated rather than assuming that each system reaches its design state independently.

Active systems can contain many components in series. ABCB gives illustrative reliability calculations using assigned component values, but these are simplified examples rather than universal system probabilities. Their purpose is to show how complexity and dependencies can reduce availability.

Normal building ventilation can either support or oppose the fire-mode arrangement. Recirculation can distribute smoke if the system does not change to its intended state. The design should document fan, damper and control positions for each relevant operating mode.

Commissioning and design limits

Commissioning should include an installation check, performance tests in all operating modes, repair of defects and retesting. ABCB presents this sequence because a completed installation is not necessarily a functioning system. Repeated testing may be needed to close identified defects.

Access for inspection and maintenance affects reliability over the system's life. Components that cannot be reached safely are harder to test and service. Documentation should identify required inspections and the mode in which each component is expected to operate.

Calculations carry uncertainty from the fire, geometry and flow paths. IFEG notes that smoke-spread calculations beyond the enclosure of origin are more difficult and uncertain than those within it. Results should not be given more precision than those inputs support.

Dilution alone should not be confused with every definition of smoke control. NUREG-1805's cited glossary definition distinguishes pressure- and airflow-based control from simple supply-and-extract dilution within an enclosure. Broader smoke-management classifications nevertheless include dilution as one technique.

The useful acceptance question is whether the intended space remains within its specified conditions for the required scenario and duration. That assessment includes the fire, barriers, air paths, equipment and operational state. A nominal airflow or installed vent area is only one input to that result.

Performance assessment

Performance should be assessed against the sequence of system states. Detection must select the intended mode, fans and dampers must respond, and airflow must then develop through the actual openings. Treating the final airflow as immediate would hide delays and incorrect intermediate states.

Different door positions can create different smoke-control problems. A closed door may preserve a pressure boundary, while an open door requires airflow through its opening. Pressure relief and door operation therefore need to be tested for the scenarios used in design.

Outdoor conditions can change naturally driven flows. Wind acts on available external openings, and indoor-outdoor temperature differences contribute to stack effect. The boundary conditions should reflect the building and fire scenario rather than assuming still outside air in every case.

Mechanical equipment also has dependencies. A fan needs a suitable control signal and operating condition, while a damper must reach its intended position. Verification should follow those functions through to airflow at the protected boundary instead of stopping at an electrical signal.

Exhaust capacity and fire size must remain compatible. ABCB warns that smoke-control performance can be compromised if sprinklers fail to limit the fire as assumed. A larger smoke source may exceed the exhaust arrangement or damage exposed components.

System complexity should be justified by the required function. Additional components can provide necessary zone control, but they also create more possible failure points. Monitoring and regular operation can help reveal faults that would otherwise remain hidden until a fire.

Maintenance should preserve both airflow hardware and the physical building boundary. Later alterations to partitions, doors, ducts or ventilation can change the smoke-control zones. Those changes require comparison with the design basis even when the original fans remain in place.

Acceptance evidence should connect calculated conditions with commissioning observations. The record should identify the protected space, scenario, operating mode, door assumptions and test results. This makes clear which performance was demonstrated and which conditions remain analytical assumptions.

A pressure measurement and a layer-height measurement answer different questions. The first helps establish flow direction across a boundary, while the second describes smoke accumulation in a volume. The acceptance record should use observations that correspond to the actual design objective rather than treating either as a universal indicator.

Reliability evidence should retain the distinction between illustrative calculations and observed performance. ABCB's generic series-system values demonstrate the effect of assumptions about complexity. They do not replace inspection, functional testing or knowledge of the installed components. Where those observations are unavailable, that limitation should remain visible rather than being replaced with a generic availability value. Performance claims should identify which parts rest on calculation and which parts rest on testing.

See also

• Fire safety strategy
• Jet fan ventilation
• Protected escape route

References

• Australian Building Codes Board. (2020). Fire safety verification method data sheets (Version 1.1). Author.
• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Cooper, L. Y. (2002). Smoke and heat venting. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
• U.S. Nuclear Regulatory Commission. (2005). Fire dynamics tools: Quantitative fire hazard analysis methods for the U.S. nuclear regulatory commission fire protection inspection program (NUREG-1805). Author.