Smoke Control in High-Rise Buildings

India’s skyline is rising faster than the engineering discipline required to keep its occupants safe during fire events. While fire suppression, detection and pumping systems have matured over time, smoke control remains one of the weakest and least understood aspects of high-rise life safety. In most towers above twenty floors, pressurisation systems, lobby protection, shaft sealing, damper control and air pathways do not behave as intended during real emergencies. What works during approval testing collapses when a live fire introduces heat, pressure differentials, leakage, stack forces & unpredictable human behaviour.

This paper examines why high-rise smoke control fails, how airflow actually behaves inside tall buildings during fire, and what engineering methods can create systems that remain stable under dynamic conditions. The emphasis is on quantifiable airflow, realistic leakage, pressure management, fan behaviour under load and a unified fire mode logic. By shifting from rule-based design to performance-based engineering, India can create buildings where smoke movement becomes predictable and evacuation becomes survivable.

Introduction: When Vertical Growth Outpaces Smoke Engineering

India’s cities are transforming. Buildings with heights of one hundred, one hundred and fifty or two hundred metres are becoming the norm in metropolitan regions. The industry has gradually improved its approach to water-based firefighting systems, detection networks and alarm integration. Yet one part of the life safety chain remains dangerously underdeveloped: smoke control.
Smoke, not flame, is responsible for the majority of fire-related fatalities worldwide. It reduces visibility, disorients occupants, creates toxic environments, increases temperature rapidly and obstructs evacuation routes. Tall buildings amplify these risks through pressure differentials, vertical air movement and complex internal pathways. The physics inside a high-rise during a fire is far more complicated than the physics inside a mid-rise or stand-alone structure.
In most Indian projects, smoke control still begins and ends with selecting a fan, adding a duct and showing a pressurisation arrow on a drawing. The gap between this treatment and the real behaviour of smoke is profound. A smoke system is not a product. It is an engineered response to airflow, leakage, heat, stack effect and human movement.
This article explores what high-rise smoke engineering actually requires. It explains why most systems fail under load, identifies the engineering mistakes commonly seen across India and presents a performance-based framework to design systems that maintain safe, predictable behaviour during real fire conditions.

1.Understanding Smoke Movement in High-Rise Buildings

Smoke does not move because a fire is burning. Smoke moves because pressure governs its direction. Tall buildings create complex pressure environments that can either control or accelerate smoke spread.

Stack effect and its influence
Stack effect occurs when differences in temperature between indoor and outdoor air generate upward or downward pressures inside shafts, staircases and lift cores. In high-rise buildings, even a two-degree temperature difference can produce significant vertical airflow. This airflow can overpower pressurisation fans if not managed correctly. Stack effect can:

• Pull smoke into vertical shafts.
• Reverse intended airflow from lobbies to corridors.
• Reduce pressurisation on upper floors.
• Intensify smoke rise in winter conditions or fall in summer depending on AC loads.

Large towers require pressure zoning and shaft sealing because a single continuous shaft can become a powerful chimney during fire.

Leakage paths that dominate airflow
Real buildings are not airtight. Common leakage points include:

• Door undercuts.
• Gaps around door frames.
• Lift landing door leakage.
• Cable risers and electrical shafts.
• Plumbing shafts and unsealed gaps.
• Imperfectly installed fire-stopping.

Even high-quality doors leak between fifteen and twenty-five cubic metres of air per hour at twenty-five Pascal. When many doors open simultaneously during evacuation, leakage volume becomes large enough to collapse pressurisation unless fan capacity includes realistic allowance for these pathways.

Fan performance under actual loading
Fan curves shown in product catalogues reflect ideal conditions. In real fire mode, fans face:

• Higher static pressures than expected.
• Blocked pathways due to closed or half-open dampers.
• Voltage fluctuations while running on DG supply.
• Reduced airflow when doors open simultaneously.
• Changes in connected duct resistance as temperatures rise.

A fan rated at ten cubic metres per second may deliver half that under real fire conditions if design assumptions do not match site realities.

Door dynamics and evacuation behaviour
People evacuate through staircases. Doors remain open longer than designers assume. Every time a door opens, pressure inside the staircase collapses momentarily. NBC requires one metre per second airflow across an open door to prevent smoke backflow into stairwells. Most high-rise designs do not size fans for this condition. As a result, smoke penetrates protected routes, reducing tenability and causing panic.

Interactions between HVAC, exhaust, pressurisation and dampers
High-rise buildings contain multiple air systems. If they do not switch correctly into fire mode:

• AHUs may continue to recirculate smoke.
• Return air risers may become smoke pathways.
• Lobby pressurisation fans may fight against smoke exhaust fans.
• Dampers may fail in unintended positions.
• Lift shaft ventilation may pull smoke upward rapidly.

The complexity of air movement inside a tall building during fire cannot be controlled by isolated components. It requires coordinated control.

2. Why Smoke Control Systems Fail in Indian High-Rises

After observing and reviewing many high-rise projects, the causes of failure are consistent. They are not due to lack of equipment but due to improper engineering.

Fan sizing based on static pressure instead of airflow demand
Designers often select fans to produce twenty to fifty Pascal inside staircases. Pressure is not the engineering target. Airflow is. The real requirement is maintaining one metre per second across an open door. For a one metre by two metre door, this equals about two cubic metres per second. With two doors open, this doubles. Most installed fans do not provide this airflow, especially when leakage loads and pressure losses are included.
Designs assume airtight staircases and lobbies. Actual buildings leak significantly. Without quantifying leakage, fan sizing becomes inaccurate. Leakage must be part of design input, not a site surprise.

Absence of pressure relief
Pressurisation without relief is unsafe. If air is pumped into a closed staircase without exit paths:

• Pressures can climb above one hundred Pascal.
• Doors become extremely difficult to open.
• Airflow direction reverses once a door suddenly gives way.
• The staircase becomes unfit for evacuation even if no smoke is present.

Pressure relief dampers ensure that the system remains stable when fans operate at higher capacities.

Misaligned sequence of operations during fire mode

A smoke control system is not made of components. It is made of timing. When AHUs do not shut down before pressurisation starts or when dampers close slower than expected, the entire smoke pathway changes. Incorrect sequencing leads to:

• Pressurisation air pushed into wrong zones.
• Return air risers carrying smoke across floors.
• Lift shafts becoming unintentional exhaust paths.
• Exhaust fans overpowering pressurisation fans.
• Inconsistent pressures floor to floor.

A building where systems do not respond in the correct sequence will not behave predictably during a fire.

Overreliance on CFD without grounding in physical leakage

CFD models often assume perfect sealing, uniform leakage, constant fan performance and fully closed doors. These assumptions rarely hold. CFD is a tool, not a substitute for engineering. When CFD is used without field leakage data or realistic boundary conditions, its predictions fail in practice. Many smoke control systems are certified without testing doors in open positions, without simulating wind pressure, without measuring actual airflows, or without observing smoke movement. A system that appears compliant during inspection may fail within minutes during a real fire if commissioning has not tested dynamic behaviour.

3. Designing Smoke Systems Using a Performance-Based Framework

High-rise smoke control must move from equipment selection to engineered performance. The following framework creates systems capable of stable behaviour during fire.

Engineering for leakage as the primary load
The first step is to quantify or estimate leakage correctly. Leakage is not an accessory input. It is the main driver of fan size.
Typical values:

• Staircase leakage: 1.5 to 2.5 cubic metres per minute per square metre of surface area.
• Door leakage: fifteen to twenty-five cubic metres per hour at twenty-five Pascal.
• Shaft leakage: varies but must be assumed conservatively if not tested.

When leakage is the starting point, fan selection becomes realistic rather than theoretical.

Fan selection based on one metre per second airflow across open doors

Pressurisation pressure is a result. The design target is the airflow needed to prevent smoke incursion. The calculation is simple in principle. Determine the largest number of doors that may be open simultaneously during evacuation. Determine their combined area. Multiply by one metre per second. Add leakage allowance. Add safety factor. The final fan capacity will be much higher than traditional sizing. Only then will the system perform during real evacuation.

Managing stack effect using pressure zoning and compartmentalisation
In towers above ninety metres, single continuous shafts cannot maintain stable pressure. Pressure zoning must be introduced through mechanical floors, sealed breaks or lobby-based compartmentalisation.

Effective zoning solves three problems:

• It limits upward or downward airflow due to stack effect.
• It ensures fans operate within manageable static pressures.
• It keeps evacuation floors from influencing pressure on non-fire floors.

Engineering sequence-of-operation for predictable behaviour

The fire mode control logic must be engineered with the same seriousness as hydronic design or electrical selectivity. The key steps are:

• AHUs shut down before pressurisation fans start.
• Return air dampers must fully close within ten to twenty seconds.
• Staircase pressurisation fans start immediately after FA signal.
• Lift shafts must be isolated before pressurisation ramps up.
• Smoke exhaust should begin only after pressurisation stabilises.
• BMS, FA and HVAC must share verified signal paths.

The reliability of smoke control depends more on this sequence than on the fan itself.

Importance of pressure relief for safe evacuation

Pressure relief dampers serve two purposes:

• They prevent excessive pressure build-up.
• They maintain directional airflow through doors.

Relief must be positioned such that air escapes into neutral or safe zones rather than unsafe ones. Relief must also be sized to match worst-case fan output.

Using multiple pressure sensors for accurate fan modulation
One sensor on one floor is insufficient. A high-rise needs three reference points.

• A lower floor sensor to detect stack-induced pressures.
• A mid-level sensor to measure zone performance.
• An upper floor sensor to adjust fan output based on actual smoke pressure.

Fans equipped with variable frequency drives can use these readings to maintain stable pressure across all heights.

Designing with airflow priority instead of pressure priority

Many designs treat twenty to fifty Pascal as the only objective. But pressure can be achieved while airflow collapses. If airflow drops, smoke enters despite adequate pressure. The system should be designed such that airflow direction and quantity remain the primary engineering criteria.

4. Solution Techniques for Specific High-Rise Challenges

Different building types require different smoke strategies.

Residential towers

• High occupant load and slow evacuation mean many doors open at once.
• Staircase pressurisation must be exceptionally stable.
• Lift lobby pressurisation prevents corridors from becoming smoke reservoirs.

Commercial towers

• Large AHU systems can rapidly distribute smoke if not isolated.
• Pressurised lobbies reduce smoke incursion from office floors.
• Quick return air isolation is crucial.

Hotels and malls

• High fuel load and variable occupancy require zoned smoke extraction.
• Atrium smoke reservoirs must be engineered with extraction based on expected heat release rates.
• Multiple fire compartments must work without interfering with evacuation routes.

Hospitals

• Horizontal evacuation is common.
• Patients cannot always move independently.
• Corridor pressurisation must prioritise equipment safety and visibility.
• Smoke entry into operation theatres or ICUs becomes life-threatening within minutes.

5. Commissioning: Where Theory Meets Physics

A smoke control system must demonstrate its performance under real conditions. Commissioning should verify behaviour, not merely installation.

A true commissioning test includes:

• Airflow measurement across open doors.
• Pressure mapping across all floors during fire mode.
• Door force testing to ensure ease of evacuation.
• Multi-door open scenarios.
• Cold smoke tests to visualise flow direction.
• Time measurement for AHU shutdown and damper closure.
• Lift shaft behaviour during pressurisation.
• Fan curve verification under load.

Commissioning must be repeated annually because system performance drifts with time.

6. Lifecycle Performance: Maintaining Smoke Control Reliability
Smoke systems degrade with use and environmental exposure.

Common degradation causes include:

• Door seals hardening or shrinking.
• Unsealed shafts emerging due to maintenance works.
• Dampers collecting dust and failing to close properly.
• Pressure sensors drifting.
• Fan belts wearing out.
• VFD settings altered during maintenance.
• Fit-out modifications introducing new leakage paths.

Smoke control requires periodic recalibration. The system that performs well during day-one commissioning may not behave the same after five years if not maintained with engineering oversight.

Conclusion: Buildings That Breathe Safely Save Lives

The future of Indian high-rise safety lies not in more powerful equipment but in better engineering of airflow, pressure and sequence-of-operation. Smoke control systems fail when they are designed around assumptions instead of real behaviour. They succeed when they are engineered with a clear understanding of leakage, stack effect, door dynamics, pressure relief and coordinated system response.
Pressurisation and smoke control must evolve into a science-based discipline where every component supports a predictable evacuation environment. Once airflow becomes the engineering priority and dynamic commissioning becomes a standard practice, India’s towers will transform from vulnerable structures into resilient life-safety systems.
Smoke is unforgiving. High-rise buildings must be engineered to match its unpredictability with precision, stability and performance that endures through decades of operation.This evolution also demands responsibility across the entire building lifecycle. Smoke control performance cannot be assumed to remain constant as buildings age, internal layouts change, and systems drift from their original calibrated state. Periodic performance testing, pressure mapping, and verification of airflow paths must become routine engineering practices rather than exceptional exercises. Designers, developers, and facility teams must recognise that smoke control is not a static installation but a living system. When maintained through disciplined engineering oversight, these systems preserve tenable evacuation routes and ensure that life safety performance endures long after construction is complete.

About the Author

Madhava Narasimha Murthy Nedunuri is a senior MEP leader with two decades of experience delivering complex high-rise, township, mall, hospital, hotel, and data-center projects across India. He began his career with engineering roles in IL&FS, Shapoorji Pallonji, HCC, and Bhartiya Urban (formerly Urbanac), progressing into strategic project leadership positions where he shaped design standards, execution quality, and safety culture. His work blends deep technical understanding with a systems-thinking approach to MEP integration, hydraulic performance, fire engineering, and sustainability. Known for his clarity in engineering logic and his commitment to mentoring teams, he continues to contribute to industry conversations through technical writing, thought leadership, and applied field insights.