Stairwell Pressure Differential Systems (PDS)

Pressure Differential Systems (PDS) play a critical role in maintaining tenable conditions within protected escape routes and firefighting access routes during a fire event. By controlling airflow and establishing pressure differentials between spaces, these systems are designed to prevent the ingress of smoke into critical areas such as stairwells, lobbies, and firefighting shafts.

The importance of Pressure Differential Systems is closely aligned with the functional requirements of Part B, Schedule 1 of the Building Regulations 2010, particularly B1 and B5.

B1: Means of Warning and Escape

Under Requirement B1, protected escape routes must remain tenable and usable throughout the evacuation period. Pressure Differential Systems achieve this through two key and complementary performance criteria:

Closed Door Condition – Pressure Differential

When doors to protected spaces (e.g. stairwells or lobbies) are closed, the system maintains a positive pressure differential between the protected space and the adjacent fire-affected area. This pressure differential acts as a barrier to smoke ingress, limiting leakage through door gaps and construction tolerances. Design pressures are carefully controlled to balance effective smoke exclusion with acceptable door opening forces, ensuring doors remain operable for occupants. This condition represents the steady-state phase of evacuation, where doors are predominantly closed and the system maintains protection under stable conditions.

Open Door Condition – Airflow Velocity

During evacuation, doors will inevitably be opened—often simultaneously across multiple levels. In this condition, pressure alone is no longer sufficient to prevent smoke spread.

The system must therefore generate sufficient airflow velocity through open doorways to oppose smoke movement. This airflow establishes a directional flow from the protected space into the fire-affected space, preventing smoke ingress.

The requirement to maintain airflow with multiple doors open concurrently reflects real evacuation scenarios in high-rise and complex buildings, where occupant movement is neither sequential nor controlled.

Together, these performance modes ensure that escape routes remain protected across both static (closed door) and dynamic (open door) conditions. The system response time and transitional control between these states is a critical aspect of PDS design and must be demonstrated during commissioning.

B5: Access and Facilities for the Fire Service

Requirement B5 introduces a more demanding operational scenario, where systems must support firefighting activities following initial evacuation. Once the Fire and Rescue Service (FRS) enters the building, the conditions within the fire compartment and adjacent areas are typically more severe:

• Smoke layer temperatures are significantly higher, increasing buoyancy effects and the potential for smoke spread
• Doors may be held open for extended periods, particularly at access levels and firefighting shafts
• Firefighting operations may introduce additional airflow disturbances, including hose lines and forced entry

Under these conditions, Pressure Differential Systems must operate beyond evacuation-phase performance and provide:

Sustained Airflow Under Open Door Conditions

Systems must maintain robust airflow through permanently open doors, rather than transient opening events. Airflow must be sufficient to resist higher-temperature, more buoyant smoke layers, which present a greater driving force than during early-stage fires.

Stability Under Adverse Conditions

The system must remain stable under changing boundary conditions, including multiple open doors and variable leakage paths. Fan performance, control strategy, and system response must ensure consistent pressurisation without collapse or reversal of airflow.

Protection of Firefighting Shafts

Firefighting stairs, lobbies, and shafts must remain relatively smoke-free to enable safe access and operational staging for fire crews. This typically requires higher duty systems and more robust design considerations than those required solely for means of escape.

Integrated System Performance

In practice, Pressure Differential Systems must be designed to accommodate both evacuation and firefighting phases, each with distinct and sometimes competing requirements:

• Controlled pressure differentials to maintain door operability during evacuation
• Sufficient airflow capacity to protect with multiple doors open
• Robust operation under elevated temperatures and extended running conditions.

This dual requirement reinforces the need for carefully engineered, fully coordinated systems, where the fire strategy is aligned with fan selection, control philosophy, leakage assumptions, and the building interface.

Pressure Differential Systems should be selected, configured, and implemented in coordination with the project fire strategy, with system performance based on defined design conditions and verified during commissioning.

Pressure Differential Systems can be applied to provide protection in the following locations, among others:

• Stairwells with or without protected lobbies
• Fire-fighting stairwells (always with lobbies)
• Corridors and hallways
• Evacuation tunnels
• Fire-fighting elevators
• Evacuation elevators

The protected areas each present different engineering challenges associated with their use during evacuation of the building or during fire-fighting.

Differential pressure systems consist of three basic components:

• Air supply and pressure relief in the pressure chamber.
• Differential pressure control between protected and unprotected space.
• Exhaust air path for pressure relief when door(s) are open.

TROX X-FANS had these basic components, including supply air fans, smoke extraction fans, control technology, opening systems, differential pressure sensors and frequency converters for the control of the fans tested in accordance with EN 12101-6:2022 (specification of differential pressure systems and kits) in cooperation with the Materials Testing Institute (MPA) Braunschweig and passed this test.

Likewise, tested kits according to Annex A, the hot gas control damper according to Annex B and the operation of smoke and heat exhaust fans with variable speed frequency converters at elevated temperature according to Annex C were tested – this test was also passed.