Pressure differential systems use controlled air pressure and airflow to resist smoke entering protected escape and firefighting routes. They may protect stairs, lobbies, corridors, firefighting lift shafts or evacuation spaces, but their performance depends on the complete building arrangement rather than the supply fan alone.
This guide explains how stairwell and lobby pressurisation systems are normally coordinated, where fire-rated ductwork may form part of the system, and what information is needed before equipment or ductwork is selected. The final design must be completed and verified by competent smoke-control specialists using the project fire strategy.
What is a pressure differential system?
A pressure differential system, often abbreviated to PDS, supplies air to a protected space so its pressure is higher than an adjacent fire-affected or unprotected space. With doors closed, the pressure difference is intended to resist smoke leakage through gaps. When specified doors are open, the system is intended to produce airflow from the protected route towards the unprotected area.
The system does not simply “blow air into a stair”. It requires a coordinated supply-air path, an air-release route, suitably controlled fans and dampers, appropriate door behaviour, automatic controls, reliable power and on-site acceptance testing.
Spaces that may be protected
| Protected space | Typical objective | Important interfaces |
|---|---|---|
| Escape stair | Help keep the stair tenable while occupants escape. | Stair doors, final exit, supply-air distribution, pressure relief and adjacent lobbies. |
| Firefighting stair and lobby | Support protected access to the fire floor and firefighting bridgehead. | Lobby doors, firefighting lift, fire main, air release and fire-service controls. |
| Firefighting lift shaft | Limit smoke entry into the lift shaft and associated protected route. | Lift doors, connected lobby, shaft leakage, fan arrangement and lift controls. |
| Evacuation stair, lift and lobby | Protect spaces used for assisted evacuation, subject to the approved evacuation strategy. | Refuge area, lift operation, lobby doors, smoke-control measures and communication systems. |
| Protected corridor or specialist space | Maintain tenable conditions or protect smoke-sensitive areas where the fire strategy requires it. | Compartment doors, leakage paths, air release, building use and operating sequence. |
The protected spaces and system objective must be agreed at the start of the design. Protecting means of escape, supporting firefighting operations and protecting property can lead to different performance requirements.
How does the system typically operate?
Following the relevant fire or smoke signal, the pressure differential system normally brings its supply and air-release components into their defined fire positions and starts the required fans. The controls then regulate pressure or airflow as doors open and close.
- Doors closed: the system maintains the required pressure relationship while keeping doors usable.
- Relevant doors open: sufficient airflow should pass from the protected space towards the fire-affected area, subject to the approved design scenario.
- Doors close again: the system must respond without creating excessive pressure or preventing safe door operation.
- Air release: air entering the unprotected area must have an effective route to outside, either naturally or through powered extract.
The actual fire-floor selection, doors assumed open, fan sequence, duty/standby arrangement, pressure relief and firefighter override must be recorded in the approved cause-and-effect matrix. A generic sequence should not be applied to every building.
Passive and powered air release
Supplying air to a stair or lobby is only one side of the airflow path. The displaced air needs a suitable release route from the unprotected area.
- Passive air release: may use controlled natural openings, facade ventilators or smoke-control dampers connected to a suitable shaft.
- Powered air release: may use smoke-control fans and associated smoke-extract ductwork or shafts to discharge outside.
If the release route is too restricted, incorrectly zoned or affected by wind, stack effect or closed dampers, the required airflow may not be achieved. If it is excessive or poorly controlled, door forces and pressure stability can also be affected.
Which standards and guidance are relevant?
| Subject | Likely reference | Practical purpose |
|---|---|---|
| PDS design and verification | BS EN 12101-13:2022 | Design objectives, calculations, installation, acceptance testing, routine testing and maintenance. |
| PDS kits and components | BS EN 12101-6:2022 | Performance requirements for pressure differential system kits. |
| Natural air-release ventilators | BS EN 12101-2 | Product performance for natural smoke and heat exhaust ventilators. |
| Powered smoke-control fans | BS EN 12101-3 | Product performance for powered smoke and heat control ventilators where applicable. |
| Smoke-control ducts and dampers | BS EN 12101-7 and BS EN 12101-8 | Product standards for smoke-control duct sections and smoke-control dampers. |
| Fire-resisting ventilation ductwork | BS EN 1366-1 / BS EN 13501-3 | Potentially relevant where supply-air ductwork must maintain compartmentation or operate during fire. |
| Multi-compartment smoke extract ductwork | BS EN 1366-8 / BS EN 13501-4 | Potentially relevant to powered air-release ductwork passing through another fire compartment. |
| Building provisions | Approved Document B or the applicable national guidance | Identifies relevant building provisions for escape, firefighting and evacuation arrangements. The jurisdiction and applicable edition must be confirmed. |
BS EN 12101-13:2022 and BS EN 12101-6:2022 replaced the older BS EN 12101-6:2005 route. Older project specifications may still refer to the withdrawn edition, so the intended design basis should be clarified rather than silently assumed.
Where fire-rated ductwork fits into the system
A pressure differential system may contain several different duct duties. They should not automatically be given the same classification.
- Supply-air ductwork: carries relatively clean air to the protected stair, lobby or shaft. Its fire-resistance requirements depend on its route, compartment crossings and required operating period.
- Powered air-release ductwork: may carry hot smoke from the fire floor and can require smoke-control duct evidence appropriate to its route and application.
- Relief or transfer paths: may incorporate tested smoke-control dampers, ventilators or shafts rather than conventional ventilation ductwork.
- Builder’s work shafts: require their own construction and fire-resistance evidence and should not automatically be treated as equivalent to tested metal duct sections.
The fire strategy and smoke-control design should state the function, classification, duration, pressure, orientation, fire exposure and insulation requirement for each duct route. The selected ductwork construction must then remain within the supporting test, classification or assessment evidence.
Information required before pricing or manufacture
- Fire strategy, evacuation strategy and smoke-control design report.
- PDS concept, calculations and defined protected spaces.
- System class and design scenarios selected by the competent designer.
- Approved cause-and-effect matrix and fire-alarm interfaces.
- Air-supply and air-release routes, fan duties and control philosophy.
- Plans, sections, risers, shaft dimensions and duct routes.
- Door schedule, opening direction, closer characteristics and leakage assumptions.
- Required duct and damper classifications, durations and installation details.
- Fire-stopping, support, access, inspection and maintenance requirements.
- Power supplies, duty/standby equipment and failure-mode requirements.
- Acceptance-testing method, witnessing responsibilities and handover deliverables.
Design influences that cannot be ignored
- Building height, shaft arrangement and leakage distribution.
- External wind pressure and the location of air intakes and discharge points.
- Stack effect caused by internal and external temperature differences.
- The number, size and opening sequence of doors.
- Door closers, seals and maximum acceptable opening force.
- Automatic sprinklers and other fire-protection measures where relevant to the selected system basis.
- Changes made to partitions, doors or ventilation after the original design.
These factors are why a fan airflow rate copied from another project is not a safe design basis.
Acceptance testing and commissioning
The completed system should be tested in the building under the defined scenarios. Depending on the approved design, verification may include closed-door pressure, open-door airflow, door-opening force, response as doors open and close, fan and damper operation, duty/standby changeover, alarm interfaces, manual controls and fault responses.
Testing should cover the relevant floor positions and operating combinations rather than demonstrating performance at one convenient location. Results, instruments, system settings, deviations and final accepted configuration should be recorded for handover and future maintenance.
Common pressurisation mistakes
- Selecting fans before leakage paths and design scenarios are established.
- Providing supply air without a coordinated release route.
- Achieving pressure at the expense of excessive door-opening force.
- Ignoring final-exit doors or doors temporarily held open during use.
- Using ordinary ventilation ductwork where continued fire operation requires supporting evidence.
- Failing to define whether extract ductwork is single- or multi-compartment.
- Mixing damper, duct or protection-system components outside their approved evidence.
- Commissioning components individually without proving the complete operating sequence.
- Changing doors, partitions or ventilation without reviewing the PDS design.
Request a ductwork scope review
Send the fire strategy, PDS report, drawings, fan schedule and ductwork requirements for an initial review of the ductwork scope. This can help identify missing information and establish an appropriate basis for fire-rated manufacture, insulation and installation.
Commercial manufacture and installation enquiries submitted through EN1366.co.uk are handled by DuctFix.
Important guidance note
This page provides general coordination guidance only. Pressure differential and smoke-control systems must be designed by competent specialists using the project fire strategy, applicable legislation and guidance, current standards, product evidence, calculations and an approved cause-and-effect matrix. EN1366.co.uk does not replace the fire engineer, PDS designer, smoke-control specialist, building-control body, approving authority or project consultant.
Source basis: BS EN 12101-13:2022, BS EN 12101-6:2022, relevant parts of BS EN 12101, BS EN 1366-1, BS EN 1366-8, BS EN 13501-3, BS EN 13501-4 and Approved Document B for England. Requirements differ by UK jurisdiction and project. Always check the current applicable documents. This page is an original practical summary and does not reproduce or replace those publications.
Guidance review: September 2026. This is an original practical summary and does not reproduce or replace the relevant standards or project evidence. How this guidance is reviewed.