💨 HVAC

Smoke Exhaust Calculator

Design smoke exhaust and smoke control systems for buildings. Calculate smoke exhaust airflow, fan sizing, and shaft design per NBC 2016 and NFPA 92.

📐 Standard: NBC 2016 Part 4 / NFPA 92
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Smoke Exhaust Calculator Calculator
Reference: NBC 2016 Part 4 / NFPA 92
💨 HVAC
Design smoke exhaust and smoke control systems for buildings. Calculate smoke exhaust airflow, fan sizing, and shaft design per NBC 2016 and NFPA 92.
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ℹ️ About This Calculator

Smoke control systems protect evacuation routes by maintaining a smoke-free layer above the occupied zone during a fire. The smoke exhaust calculator determines the required exhaust fan capacity based on fire size, plume entrainment, and design interface height. Applicable to car parks, atriums, corridors, and pressurised stairwells per NBC 2016 Part 4 and NFPA 92.

NBC 2016 Part 4 mandates mechanical smoke exhaust for: underground car parks, atria > 10 m high, corridors in high-rise buildings, basements, and enclosed shopping centres. Smoke exhaust fans must be rated for 400°C for 2 hours (F400 class). Fan controls: smoke detectors trigger the system; zone dampers open in the fire zone. Smoke shafts (pressurised exhaust shafts) serve multiple floors of high-rise buildings.

📐 Smoke Exhaust Design (Plume Method)

NBC 2016 Part 4 / NFPA 92

Fire Heat Release Rate (HRR):
  Q_fire = 1.5–5 MW (design fire size; car park: 1.5–3 MW)

Axisymmetric Plume (NFPA 92):
  If z > z_l (above flame height):
    ṁ = 0.071 × Q_c^(1/3) × z^(5/3) + 0.0018 × Q_c
  Smoke layer depth d = H − z_interface (≥ 1.8 m clear height)

Car Park Simplified (NBC 2016):
  Q = 10 ACH × V_car_park (minimum 10 air changes/hour)
  Or Q = 6 L/s per m² of floor area (natural draft)
  Jet fans for car parks > 2000 m²

Make-up air: 85–90% of exhaust flow (prevent starvation)

🧮 Worked Example

An underground car park measures 60 m × 40 m × 3 m, giving a volume of 7200 m³. Applying the NBC simplified method at 10 air changes per hour: Q = 10 × 7200 = 72,000 m³/h = 20 m³/s. Checked against the area-based method at 6 L/s per m² of floor area (2400 m²): Q = 2400 × 6 = 14,400 L/s = 14.4 m³/s. The higher value, 20 m³/s, governs the exhaust fan selection (F400/2h rated). Make-up air is then provided at 85–90% of this exhaust rate, approximately 17–18 m³/s, supplied at low level to prevent the extract system from starving.

❓ Frequently Asked Questions

What air change rate is required for car park smoke exhaust? +
NBC 2016 minimum: 10 air changes per hour (ACH) for enclosed car parks (mechanical ventilation required). For fire mode (smoke exhaust): 10 ACH or 6 L/s/m² floor area, whichever is greater. Typical design: 8–10 ACH normal ventilation (CO control) doubles as smoke exhaust during fire. Jet fan systems (for large flat car parks): designed to direct smoke plumes towards extract points, not just dilute.
What temperature rating do smoke exhaust fans require? +
F400/2h rating: fan must operate at 400°C continuously for 2 hours. F300/1h: fan operates at 300°C for 1 hour. NBC 2016 and EN 12101-3 define classifications. F400/2h is the standard for all smoke exhaust in occupied buildings. The fan manufacturer must provide a certificate of compliance. Standard HVAC fans are NOT suitable for smoke duty – they will fail when exposed to hot smoke.
How is a stairwell pressurisation system different from smoke exhaust? +
Smoke exhaust removes smoke from the fire zone. Stairwell pressurisation keeps smoke OUT of the escape staircase by maintaining a higher pressure in the stair shaft than in the floor corridor. If the stair door opens: pressure difference prevents smoke ingress. The pressurisation fan supplies fresh air to the stair shaft. Pressure differential: 25–50 Pa (BS EN 12101-6). Both systems work together in high-rise buildings.
Do I need make-up air for smoke exhaust to work? +
Yes. If you extract smoke without providing replacement air, negative pressure develops and the system chokes (extracting very little smoke despite the fan running). Make-up air must be provided at approximately 85–90% of the exhaust volume, supplied at low level (below the smoke layer) so clean air replaces extracted smoke. Natural make-up air (open external doors) is acceptable in some configurations; mechanical supply is used for enclosed spaces.
What triggers the smoke exhaust system? +
Activation sequence: smoke detector or heat detector in the zone triggers alarm; BMS (Building Management System) or dedicated fire control panel sends signal to: (1) start smoke exhaust fan, (2) open zone smoke dampers, (3) close normal ventilation dampers, (4) start make-up air supply fan, (5) pressurise stairwells. Manual override at fire brigade control panel. System must remain active for 2 hours minimum (battery backup for controls).

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →