Post-Fire Smoke Purge Systems: Understanding Their Requirements

In the New York City Building Code, a post-fire smoke purge system is defined as a natural or mechanical ventilation system intended to remove smoke from indoor areas once a fire has been extinguished. It is important to note that post-fire smoke purge systems are not designed to fight an active fire, but rather to help resume normal activities as quickly as possible once a fire has been controlled. Therefore, post-fire smoke purge is not considered a life safety system and is not subject to same requirements as smoke control systems.

The NYC Building Code covers post-fire smoke purge systems in depth in Chapter 9, Section 916, establishing the minimum requirements for their design and installation. The building code also lists the four cases where these systems are mandatory:

• High-rise buildings that are subject to Section 403 of the NYC Building Code.
• Buildings where any floor has as area of more than 50,000 square feet.
• Buildings with spaces located at least 100 feet away from natural ventilation openings, which are defined as operable windows and doors with at least 5% of the total floor area.
• Locations with high-piled stock or rack storage containing combustible materials, as defined by the NYC Fire Code.

Regardless of the type of occupancy, all post-fire smoke purge systems must exhaust the air outside the building, in a safe area, and according to the requirements of the NYC Mechanical Code. The air extracted by post-fire smoke purge systems must not be recirculated to other building areas under any circumstances.

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Post-Fire Smoke Purge in Multifamily Residential Occupancies (R-2)

The NYC Building Code accepts two different methods for sizing post-fire smoke purge systems that will serve R-2 occupancies: stair ventilation or corridor ventilation. It is only necessary to deploy one of them to meet code requirements.

If stair ventilation is used, a fan system must be deployed at the top of all enclosed exit stairs. The system must be reversible, capable of both injecting fresh air and exhausting indoor air with smoke. To determine the required capacity of the fan system, the two values below must be calculated for the largest floor in the building. Then, the highest of the two values is used:

• 6 air changes per hour (ACH)
• 1 cubic foot per minute per square foot of floor area (CFM/ft2)

If corridor ventilation is used, each corridor must be equipped with reversible fans and ducts. The required fan system capacity is 6 ACH or 1 CFM/ft2, whichever is higher. In this case, the calculation is based on the sum of the largest apartment area and the corridor area.

In both cases, the post-fire smoke purge system must be equipped with manual controls. If a fire command center is mandatory according to section 911 of the code, it must include the post-fire smoke purge control. On the other hand, if a fire command center is not mandatory, the manual control must be included in the fire alarm panel. The control must be equipped with a graphic display that indicates the building areas being served by each post-fire smoke purge system.

If stair ventilation is used, the Fire Department personnel controls the post-fire smoke purge system by opening the doors leading from the affected floor to the staircase. If the building uses corridor ventilation, each floor must be equipped with dedicated post-fire smoke purge controls.

If an R-2 building meets either of the following conditions, the post-fire smoke purge system is no longer mandatory:

• Openable windows in every habitable room: They must have a glazed area of at least 12 square feet each, and their total area must be at least 10% of the floor area served. Each window must have an openable area of at least 6 square feet, with a total openable area equivalent to at least 5% of the floor area served. The openable area of windows must be at least 30 inches above the floor, and free from any devices that limit their opening. In addition, the windows must comply with Chapter 12 of the NYC Building Code.
• Smoke-proof enclosures: A post-fire smoke purge system is not required if all exits are built as smoke-proof enclosures compliant with the NYC Administrative Code.

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Post-Fire Smoke Purge in Other Occupancies

Post-fire smoke purge systems for occupancies that are not classified as R-2 are subject to a common set of specifications. The general design requirements are the following:

• Post-fire smoke purge can use a dedicated installation, the existing HVAC system or other openings that lead to the building exterior, as long as it has the capacity to remove smoke from occupied spaces.
• Either mechanical or natural ventilation can be used, but the system must be able to exhaust cold smoke.
• All smoke must be exhausted outdoors, at a safe location and without being recirculated to other building areas, meeting the requirements of the NYC Mechanical Code.

Exhaust Capacity

The post-fire smoke purge system for occupancies other than R-2 can be zoned and calculated based on the largest area served, where the minimum capacity is 6 ACH or 1 CFM/ft2, whichever is higher. If suspended ceilings are used, the volume above must be considered for the ACH calculation. Regardless of which value is higher, it is necessary to ensure that enough make-up air is provided through doors, windows, leakage or mechanical ventilation.

Control Requirements

Just like in R-2 occupancies, the post-fire smoke purge system must be controlled manually from the fire command center or fire alarm control unit, whichever applies for the building in question. There must be a graphic display of the building areas served by each system, and individual controls are required for each area in the case of zoned systems.

The manual controls of post-fire smoke purge systems must not override the smoke control system operations, but they must override the shutdown signal provided by the fire alarm system.

Concluding Remarks

Like any building system, the post-fire smoke purge system must be serviced regularly to ensure proper operation when needed. The NYC Building Code requires testing records to be kept on site for inspection by the Fire Department.

If you are developing a construction project in New York City, verify if a post-fire smoke purge system is mandatory. The best way to ensure that you will comply with NYC codes is hiring the services of a qualified consultant or engineering firm before proceeding with the project. Keep in mind that post-fire smoke purge systems are subject to many requirements by the NYC Building Code, Administrative Code, Fire Code and Mechanical Code.

Editors Note: This post was originally published in May 2016 and has been revamped and updated for accuracy and comprehensiveness.