Smoke control system design protects life by keeping escape routes tenable during a fire: it uses zone pressurization to keep stairs and corridors at positive pressure and exhaust to remove smoke from the fire zone. The goal is not to extinguish the fire but to hold a 12.5–50 Pa pressure difference across barriers so smoke cannot migrate into occupied escape paths. Design follows codes such as NFPA 92 and EN 12101, and every value below is a typical engineering target rather than a fixed rule.
Two Core Strategies
Pressurization pushes clean air into a protected volume so any leakage flows outward, away from occupants. Exhaust (or smoke extraction) pulls smoke out of the fire zone to lower its layer temperature and improve visibility. Most tall buildings combine both: stairs and lobbies are pressurized, the fire floor is exhausted. The protected stair usually targets 50 Pa relative to the fire floor, while a corridor target is closer to 12.5–25 Pa to avoid door-opening forces that exceed 100 N.
Pressurization vs. Exhaust at a Glance
| Factor | Zone Pressurization | Smoke Exhaust |
|---|---|---|
| Primary aim | Keep escape routes smoke-free | Clear the fire zone of smoke |
| Typical pressure | +12.5 to +50 Pa | Negative relative to surroundings |
| Air movement | Clean air in, leak out | Smoke drawn to exhaust |
| Best for | Stairs, lobbies, refuges | Atria, car parks, large halls |
| Key risk | Door-opening force too high | Make-up air path missing |
The split is not exclusive; a car park of 2,000 m² may need 10–15 air changes per hour of exhaust plus a makeup path, while a 20-storey tower needs stair pressurization held within 50 Pa even with two open doors.
Calculating the Numbers
Pressurization flow is set by leakage area, not by room volume. A stair enclosure with 0.1 m² of total leakage at 50 Pa needs roughly 0.1 × 1.3 × sqrt(50) ≈ 1.0 m³/s of supply, but designers add margin for one or two open doors, raising demand to 2–3 m³/s in tall buildings. Exhaust for a fire zone uses the smoke layer interface: keep it at least 1.8 m above the highest walking surface, so a 500 m² hall may need 6–10 m³/s of extract depending on heat release rate.
Make-Up Air and Balancing
- Supply make-up air equal to at least 80% of exhaust so the fire zone does not go strongly negative and pull smoke backward.
- Balance lobby and stair pressures so the lobby sits between the stair (higher) and floor (lower).
- Use impulse (jet) fans in car parks to drive smoke to the nearest exhaust inlet within 30–60 seconds.
- Interceptor doors and smoke curtains extend the protected volume without extra fans.
Decision Checklist: Which Method
- Choose pressurization when protecting a defined escape route such as a stair, refuge, or corridor with few openings.
- Choose exhaust when the space is large and open, such as an atrium, theatre, or basement car park over 500 m².
- Combine both when a fire floor must be cleared while the stair stays clean—the normal high-rise case.
- Add a makeup-air path whenever exhaust exceeds 5 m³/s or the zone is fairly tight.
- Trigger from a fire alarm and verify with a 50 Pa pressure test before handover.
Verification and Commissioning
A smoke control system is only proven by test. Measure the stair-to-floor pressure with one and two doors open; it must stay within 12.5–50 Pa and the door-opening force under 100 N. Run the exhaust fans and confirm the smoke layer holds above 1.8 m for the required duration. Components that are balanced to G2.5 and performance-tested before shipment give a stable platform, but the system still needs this field verification to be trusted.
Stairwell vs. Elevator Pressurization
Stairs are pressurized to 50 Pa; elevators are usually pressurized to 25–40 Pa or exhausted, depending on local code. Never pressurize a stair and an adjacent elevator lobby to the same value or the pressure path short-circuits through the shaft. Keep the elevator lobby between the stair (high) and the floor (low) so smoke moves away from both escape routes and occupants meet clean air at every door they open.
Atria and Large Volumes
Atria use exhaust, not pressurization, because they have no sealed enclosure to hold pressure. Size exhaust to keep the smoke layer 1.8 m above the highest walking surface; a 400 m² atrium with a 4 MW fire may need 12–18 m³/s of extract plus make-up air. Sprinklers lower the needed rate, so coordinate with the fire engineer before fixing the fan size and the inlet layout.
Make-Up Air Path Design
Exhaust needs a designed make-up path, not a leaky envelope. Supply make-up equal to at least 80% of exhaust through grilles placed low and away from the exhaust inlet, so smoke is swept to the extract rather than recirculated. In a car park, impulse jet fans stage on to push the layer toward the nearest inlet within 30–60 seconds, and the make-up doors must open on the same fire signal as the fans.
Failure Modes to Avoid
Common failures are a stair that exceeds 50 Pa so the door will not open, an exhaust fan with no make-up that pulls smoke backward, and a system that passes with one door but fails with two. Size the supply fan for two open doors, set the relief damper so pressure never exceeds the band, and prove the make-up path in the same test as the extract before the system is accepted.
Code Frameworks
Design references NFPA 92 for active smoke control and, in Europe, EN 12101 for fans and EN 1366 for penetrations. Local codes may set the exact pressure band and the acceptance test, so confirm the adopted edition. The physics is the same across jurisdictions; only the pass/fail numbers and the documentation format differ from one authority to the next.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What pressure should a protected stair be kept at?
A protected stair is typically held 12.5–50 Pa above the fire floor, with 50 Pa as the common upper limit for tall buildings. The pressure must hold with one or two doors open yet keep the door-opening force under 100 N so occupants can still escape safely during a fire.
Pressurization or exhaust—which do I need?
Use pressurization for defined escape routes like stairs, refuges, and tight corridors, and exhaust for large open spaces such as atria and car parks over 500 m². High-rise buildings normally combine both so the fire floor is cleared while the stair remains smoke-free.
How much make-up air is required?
Supply make-up air at no less than 80% of the exhaust flow so the fire zone does not go strongly negative and pull smoke backward through openings. For exhaust above 5 m³/s or tight zones, treat the makeup path as a designed element rather than a passive leak.
How is a smoke control system verified?
Verify by measuring stair-to-floor pressure with one and two doors open, confirming 12.5–50 Pa and under 100 N door force, then running exhaust to show the smoke layer stays above 1.8 m for the required time. This field test is required before the system is accepted.