Smoke Control Systems: Design, Commissioning and Testing Requirements

Of all the systems in a large building, smoke control is the one most likely to move a completion date. It cannot be signed off from a submittal, and it cannot be proven until the envelope is closed, the fire alarm is accepted, the elevators run and both power sources are live. Everything else standing between a project and a certificate of occupancy can usually be worked in parallel. Smoke control sits at the end of the queue by physics, and when it fails there is very little programme left in which to fix it.

This article sets out what a smoke control system is, how the different families work, where the codes require them, what the analysis behind an atrium design involves, and what the testing regime demands. Budlong delivers fire and life safety engineering alongside full MEP engineering services on California high rise, healthcare, aviation and civic projects, and most of what follows comes from watching where these systems succeed and fail on real jobs.

1. What Is a Smoke Control System?

A smoke control system is an engineered arrangement of barriers, fans, dampers and controls that manages where smoke goes during a fire. It is not a firefighting system and not a ventilation system. Its job is to keep the routes people use to leave the building, and the routes firefighters use to reach the fire, tenable for long enough. It exists because in certain geometries compartmentation cannot work alone. An atrium has no floor slabs to stop vertical smoke movement. A high rise stair is a chimney. An underground floor has no windows to open.

Smoke control is not smoke removal

This distinction is worth settling before any fee is agreed. Smoke control operates during the fire and has to maintain a defined tenable condition. Smoke removal, often called purge, operates after the fire is out so the fire department can clear the building for salvage and overhaul. Smoke removal is required in high rise buildings, but it is a firefighting convenience, not a life safety system, and it does not carry the same analysis, standby power, control listing or special inspection obligations.

Projects routinely price a smoke removal allowance and later discover the atrium geometry demanded a full engineered system. The difference in fee is significant; the difference in equipment, controls and commissioning cost is much larger. Getting this call right early is the clearest argument for bringing fire safety engineering into the team during schematic design rather than during construction documents.

2. How Smoke Control Actually Works

Every smoke control system uses one of three physical mechanisms, or a combination of them. The code recognises all three, and the choice follows from the geometry rather than from preference.

The pressurisation method

Air is supplied to hold a space at higher pressure than the space where the fire is, so that leakage through cracks, gaps and closed door edges runs from the clean side to the smoky side. This is how stairs, elevator hoistways and refuge areas are protected. The pressure difference required across a smoke barrier depends on ceiling height and on whether the space is sprinklered. The code table runs from 0.05 inches of water gauge in a sprinklered building up to 0.18 inches for an unsprinklered space with a 21 foot ceiling.

The airflow method

Air is driven through an opening at sufficient velocity that smoke cannot travel back against it. The velocity needed rises steeply with fire size, and pushing air toward a fire supplies it with oxygen. The code permits the method but constrains it tightly, and it is used sparingly.

The exhaust method

Smoke is removed from the top of a large volume fast enough that the layer accumulating below the ceiling stops descending before it reaches the people underneath. This is how atria, covered malls and other large volumes are handled, and it requires the most analysis. The code target is that the smoke layer interface stays at least six feet above the highest walking surface used for egress, for either twenty minutes or one and a half times the required safe egress time, whichever is greater.

3. The Types of Smoke Control System

Smoke control divides first into passive and active. Passive smoke control is compartmentation: smoke barriers, smoke partitions, rated shaft enclosures, and the smoke dampers that maintain those barriers where ductwork passes through them. It needs no fans and no controls beyond damper closure, and it is always the first line of defence. Active smoke control comes in four practical forms.

The atrium analysis in more detail

Atrium exhaust is the only one of the four that is genuinely a calculation rather than a specification. The engineer selects a design fire, expressed as a heat release rate and a growth rate. Fire growth is normally represented as a t squared curve, categorised as slow, medium, fast or ultrafast according to how long the fire takes to reach roughly 1,000 Btu per second, which is about 583, 291, 145 and 73 seconds respectively. Around seventy percent of the heat release is convective and drives the plume.

The plume then entrains air as it rises, so the higher the smoke layer sits, the more air is entrained and the more smoke there is to exhaust. For an axisymmetric plume rising freely, the mass flow above the flame region follows the NFPA 92 correlation in which mass flow is proportional to the convective heat release to the one third power multiplied by height to the five thirds power. The consequence is counterintuitive for owners: a taller atrium needs a larger exhaust system, not a smaller one, because the plume has further to travel and entrains more air on the way.

Two other plume geometries usually matter more. A balcony spill plume forms where smoke from a fire under a projecting balcony travels horizontally, spills over the edge and then rises, and its effective width is the opening width plus the balcony depth. A window plume forms where a compartment fire vents through an opening into the atrium. Both routinely produce exhaust rates that dwarf the free standing case, and the balcony spill plume is very often what sizes the system.

4. Which Projects Require Smoke Control

Smoke control is triggered by geometry and occupancy, not by size alone. A three storey building with an open stair connecting all three floors can be captured while a much larger warehouse is not.

Atriums

A vertical space closed at the top and connecting three or more storeys is an atrium, and it requires an engineered smoke control system. In some institutional occupancies the threshold drops to two storeys. This is by a wide margin the most common trigger, and the one most often created inadvertently by an architectural move made after the fire strategy was set.

High rise buildings

Buildings with an occupied floor more than 75 feet above the lowest level of fire department vehicle access require smokeproof enclosures for their interior exit stairways, which in practice means stair pressurisation. They also require a post fire smoke removal capability, and many designs add hoistway pressurisation as the alternative to enclosed elevator lobbies on every floor. A tall residential tower can therefore end up with three separate smoke related systems, which is why MEP solutions for high rise residential projects treat these as one coordinated scope rather than three line items.

Covered malls, underground buildings and assembly

A covered mall building with an atrium falls under the atrium provisions. Underground buildings with an occupied floor more than 30 feet below the level of exit discharge require smoke control designed to restrict the migration of combustion products, and where such a building is compartmented, each compartment needs an independent system.

Smoke protected assembly seating, which allows arenas and theatres to use the more generous egress capacity factors, depends on a smoke control system to justify the protection. Areas of refuge and enclosed elevator lobbies in some designs rely on pressurisation to remain usable. Stages above defined size and height thresholds, windowless buildings and Group I-3 detention occupancies carry their own requirements.

5. What Smoke Control Asks of Each Building System

Smoke control is described in the fire chapter of the code but built almost entirely out of mechanical and electrical equipment. That mismatch causes most of the coordination failures.

Mechanical

Air handlers serving a zoned system must be capable of one hundred percent outside air and of relieving the same volume. That is a different machine from one sized purely for a cooling load, and it changes fan selection, motor size, plant room footprint and the electrical service. Atrium exhaust fans must be rated to move smoke at elevated temperature and keep running while doing so. All of it belongs in HVAC design rather than submittal review, and HVAC load calculations matter unusually here, because the smoke mode duty and the comfort duty are different problems solved by the same equipment.

Dampers

Every point where a duct crosses a smoke barrier needs a combination fire and smoke damper, listed for leakage class and actuated so its position can be proven at the control panel. On a zoned high rise system the damper count runs into the hundreds. Each one needs power, control wiring, end switches and, critically, access for testing. A damper that cannot be reached cannot be tested, and a damper that cannot be tested is a failed inspection.

Controls, fire alarm and standby power

The smoke control panel must be listed under UL 864 with the UUKL smoke control category. A standard building automation controller cannot legally perform this function. Activation comes from the fire alarm system, and that interface is where most first pass test failures originate. The code sets hard response times: total time from command to fan operation must not exceed 60 seconds, and complete damper travel must not exceed 75 seconds. The panel also runs an automatic self test weekly. Getting the interface right is a low voltage design problem as much as a fire problem.

Smoke control equipment must also run on standby power, with transfer completing within 60 seconds of a loss of the primary source. That adds fan motors, dampers and the control panel to the generator load calculation, and generator sizing is hard to change late. Coordination between the electrical engineering package and the smoke control sequence needs to happen while the generator is still a number on a schedule.

The firefighter smoke control station

The code requires a firefighter smoke control station, normally in the fire command centre, giving the incident commander a graphic depiction of the building, manual override of every fan and damper, and confirmation of actual equipment status rather than commanded status. That distinction matters. A light showing a fan was told to start is not compliant. The station must show the fan is running, and annunciate a fault if confirmation does not arrive.

Large exhaust plenums, high level ductwork, makeup air louvres and stair pressurisation shafts all compete for the same space as structure and architecture. This is exactly the class of conflict that MEP BIM coordination and clash detection exists to catch, and it is far cheaper to resolve a shaft conflict in the model than to discover a louvre cannot be built after the facade is procured.

6. The California Layer: Title 24 Parts 2 and 9

California does not adopt the International Building Code directly. It adopts it with state amendments and publishes the result as part of Title 24 of the California Code of Regulations. Part 2 is the California Building Code and Part 9 is the California Fire Code, both maintained under the coordination of the California Building Standards Commission, with the Office of the State Fire Marshal responsible for most of the fire and life safety amendments.

The 2025 edition of both parts is based on the 2024 International Building Code and International Fire Code and took effect for permit applications submitted on or after 1 January 2026. Section 909, which governs smoke control, appears in both Part 2 and Part 9, so a project answers to the building department and the fire department against substantially the same text. In practice they read it differently, and reconciling those readings before test day rather than during it is a real part of the engineer’s job.

What the California amendments add

The California version of Section 909 keeps the structure of the model code but carries state specific detail on the rational analysis submission, on system response time and on maintenance. The maintenance clauses are numbered differently but land in the same place: dedicated systems are operated for each control sequence semiannually, non dedicated systems at least annually, and any component that has been bypassed is tested semiannually.

Special inspection in California

Smoke control appears in the special inspection chapter of the California Building Code as a listed work type. The code requires that smoke control systems undergo special inspections and tests sufficient to verify the proper commissioning of the design in its final installed condition, and the construction documents must state the procedures, methods and the specific items subject to those inspections. This is not a plan check comment. It is a construction phase obligation with a named inspector and a report the building official will look for before signing a certificate of occupancy.

Local amendments add further weight. In the City of Los Angeles the municipal code requires that all smoke control systems be tested before occupancy by authorised representatives of both the Department of Building and Safety and the Fire Department, that the report be filed with both, and that the system be retested every six months to requirements set jointly by the two departments. Assume the local layer differs in every California jurisdiction you work in. Our guide to code compliant fire safety engineering in California covers how those variations behave.

7. Healthcare, Aviation, Transit and Assembly

Four sectors deserve separate treatment because their smoke control problems are structurally different from a commercial office tower.

Healthcare

Hospitals evacuate horizontally. Patients are moved from one smoke compartment to an adjacent one on the same floor, which makes the integrity of the smoke barrier and every damper in it a direct patient safety issue. Hospital smoke control also has to coexist with pressure relationships that exist for infection control, and the two can point in opposite directions in the same corridor. California hospitals are reviewed by the Department of Health Care Access and Information rather than the local building department, which adds a separate review track. Our healthcare facilities work meets this on nearly every acute care project.

Aviation and transit

Airport terminals and transit stations combine very large volumes, high occupant loads and long egress travel distances. Terminal concourses are effectively permanent atria, often with balcony conditions above the departures hall that make the spill plume the governing case. Underground stations add the underground building provisions and independent smoke control per compartment. These projects almost always use computational fluid dynamics rather than hand correlations, because the geometry is too irregular for a simple plume model to represent honestly. Budlong’s aviation portfolio includes terminal work where exactly this analysis governed the mechanical scope.

Education and assembly

Performing arts centres, gymnasia, student unions and campus libraries pick up smoke control through atrium geometry and through smoke protected assembly seating. The seating case works in the owner’s favour, because smoke control is what permits the more generous egress capacity factors and can reduce the number of exits and the width of aisles. Make that trade early: reversing it late means redrawing the seating bowl. It comes up regularly across our education projects.

Mission critical

Data halls rarely need smoke control in the atrium sense, but the office and support blocks attached to them frequently do, and the interface between a very early warning detection system and a smoke control sequence needs care. A false activation that dumps one hundred percent outside air into a conditioned space is a genuine operational risk. See our mission critical capability for how that interface is handled.

8. Dedicated or Non Dedicated Systems

The single largest cost and risk decision in smoke control is whether the system uses its own equipment or borrows the building HVAC plant. The decision is normally made in design development and is very expensive to reverse.

The periodic testing frequencies run counter to intuition. Dedicated systems are tested more often precisely because nothing else exercises them, so a seized damper or failed actuator can sit undetected for months. Non dedicated systems get used every day, which catches hardware failures, but they hide sequence errors that only appear in smoke mode.

Most large buildings end up with a hybrid: dedicated fans for stair and hoistway pressurisation, and a non dedicated arrangement for the zoned floor system. That is defensible, but the commissioning plan then has to cover both regimes. It is the same argument as the one for early MEP coordination reducing rework in construction generally, and our fire safety solutions group makes this call jointly with the mechanical team rather than handing a requirement over the fence.

9. Testing, Special Inspection and Commissioning

This is where smoke control damages programmes. Every other system can be proven progressively. Smoke control has a component layer that can be proven early and an integrated layer that cannot be proven until nearly everything else is finished and accepted.

The stages of testing

Why the integrated test cannot be pulled forward

The integrated test measures pressure differences across barriers, door opening forces, damper positions and fan status under real conditions, and every one of those measurements depends on the building being finished. An open envelope changes the pressure regime completely. An unfinished fire alarm system cannot issue the commands. Elevators not on final adjustment cannot demonstrate recall interacting with hoistway pressurisation. Standby power that has not been load bank tested cannot demonstrate the 60 second transfer. There is no sequence in which this test happens early.

Where integrated testing standards fit

The standards published by the National Fire Protection Association give this work a formal structure. NFPA 92 governs how smoke control systems are designed and what has to be demonstrated. NFPA 3 covers commissioning of fire protection and life safety systems, and NFPA 4 covers integrated testing, including the role of an integrated testing agent who plans and leads the test across all interfacing systems. Adopting that framework voluntarily, even where the local code does not mandate it, is the most effective way to protect the programme.

Why coordinated commissioning is the mitigation

Smoke control testing overlaps heavily with mechanical commissioning, air balancing, fire alarm acceptance, elevator acceptance and generator load testing. Run as five separate exercises, each trade turns up on its own, finds a problem caused by another trade, and leaves. Run as one coordinated scope with a single test plan, the interactions are found in one pass. That is what a structured building commissioning process is built to do, and why commissioning services belong in design rather than at substantial completion.

Guidance from ASHRAE, whose smoke control handbook underpins much of the analysis methodology, and fire research from NIST, source of the plume correlations and fire models the industry relies on, make the same point in different words: a smoke control design is only as good as the assumptions that are verified in the field.

Key Takeaways

• Smoke control operates during a fire to keep egress and firefighting routes tenable. Smoke removal operates after it, for salvage. Price and scope them separately.
• Atriums connecting three or more storeys, high rise stairs above 75 feet, underground floors more than 30 feet below exit discharge, covered malls and smoke protected assembly seating are the main triggers.
• Stair pressurisation holds 0.10 to 0.35 inches of water gauge; hoistway pressurisation holds 0.10 to 0.25. Door opening force must stay under 30 pounds, so over pressurisation is also a failure.
• Atrium exhaust is sized from a design fire and a plume calculation. The balcony spill plume usually governs, and taller atria need more exhaust, not less.
• Makeup air is limited to 200 feet per minute where it can reach the plume, which drives louvre area and architectural coordination.
• The control panel must be UL 864 UUKL listed, fans must respond within 60 seconds, dampers within 75, and standby power must transfer within 60 seconds.
• Special inspection is a construction phase obligation with a named inspector and a report the building official requires before occupancy.
• The integrated test cannot be pulled forward. Schedule it backwards from the occupancy date at the start of construction and coordinate it with all adjacent commissioning activity.

10. Where Budlong Works

Smoke control is written into a statewide code but enforced locally, and the local layer is where projects are judged. Los Angeles requires witnessing by two departments and a six month retest cycle. Fire departments differ on design fire assumptions, on tenability criteria and on how much analysis they want to see before permit. Budlong maintains teams across California so that this local knowledge sits behind the submission rather than being learned on the job. More on how the practice is organised is on our company story page.

Sectors where smoke control governs the programme

Healthcare carries horizontal evacuation and competing pressure relationships. Aviation and transit carry very large volumes and spill plume geometry. Education carries assembly seating and atrium student commons. Mission critical carries the detection interface. Each brings a different failure mode to the integrated test.

11. Related Reading

12. Frequently Asked Questions