When a fire occurs inside a building, there are additional risk factors beyond getting burned by the fire itself. Smoke can also be very dangerous, since it limits visibility and can complicate evacuation. In addition, smoke contains carbon monoxide (CO), which is a highly toxic gas for humans - at concentrations above 10,000 ppm, carbon monoxide causes unconsciousness within seconds and death within minutes.
A post-fire smoke purge system has the goal of extracting smoke from indoor spaces and making it suitable for occupancy as fast as possible after the fire has been controlled. For this reason, it is not considered a life safety system like sprinklers by NYC codes, but it is mandatory in various occupancy classifications.
This article will provide an overview of the two post-fire smoke purge methods accepted by the NYC Building Code for multifamily residential buildings: stair ventilation and corridor ventilation. Deploying one of them is enough for code compliance, but they can also be used together. These systems are also required in other occupancy classifications, and the full list of requirements and specifications can be found in Chapter 9, Section 916, of the NYC Building Code.
As its name implies, a stair ventilation system uses staircases as ducts to remove smoke from indoor spaces. Openable doors connect the staircase with all levels in the building, and they are manually operated by fire department personnel to serve the floor that requires smoke to be purged.
If this type of post-fire smoke purge system is deployed, it must include all staircases in the building. Each staircase must be equipped with a reversible fan at the top, allowing both fresh air supply and indoor air exhaust. The fan must have a capacity of 6 ACH or 1 CFM per square foot, whichever is larger, based on the largest floor served by the staircase. The fan capacity is calculated based on the largest floor area to ensure it will be able to serve any other floor if affected.
The location of post-fire smoke purge controls changes depending on other fire protection requirements. In buildings where a fire command center is mandatory according to Section 911, manual controls for the post-fire smoke purge system must be installed there. On the other hand, if the fire command center is not required for the building in question, post-fire smoke purge controls must be located on the fire alarm panel. In either case, the controls must include a graphic indicating which building areas are served by each post-fire smoke purge system. This way, firefighters can easily find and open the stair doors corresponding to the level where smoke purge is required.
When this approach is used, smoke from each level is exhausted directly outdoors by reversible fans, without using staircases as smokestacks. The required duct and fan capacity in this case is also 6 ACH or 1 CFM per square foot, whichever is larger, but based on the area of the corridor plus the largest apartment in the floor. The logic is similar to that of stair ventilation systems: capacity is based on the largest apartment so the post-fire smoke purge system can serve any other apartment in the floor.
The controls must be located in the fire command center if required for the building, or on the fire alarm panel if there is no fire command center. Individual controls are required for each smoke purge system, as well as a graphic indicating the building portions served by each system.
Comparison Between Both Types of Systems
For a given building size, stair ventilation generally has a lower cost because fans and controls are consolidated into less units located above staircases. However, the system is more complex to operate because firefighters must manually open the door connecting the affected floor with the staircase used to purge smoke. On the other hand, a corridor ventilation system is easier to operate but more expensive to build, since there are more fans, each with its own ducts, electrical wiring and controls.
Regardless of the system configuration used, it must be kept under optimal conditions, with testing records on-site for review by the NYC Fire Department.
Cases Where Post-Fire Smoke Purge is Not Required
There are two cases where the NYC Building Code does not require post-fire smoke purge in high-rise multifamily buildings: when the building has a minimum openable window area as described in the Building Code, or when smokeproof enclosures are used for all exits. If the post-fire smoke purge system is omitted based on the minimum openable window area exception, the following conditions must be met:
Openable windows must be installed in every habitable room, compliant with Chapter 12 of the NYC Building Code - Interior Environment. The total window area must be at least 10% of floor area served.
Windows must have a glazed area of at least 12 square feet, with an openable area of at least 6 square feet, and without stops or devices that prevent a full opening. Thus, the minimum openable area is at least 5% of floor area served.
Windows must be installed at least 30 inches above the finished floor.
If any other NYC code requires window guards for the building in question, and these include stops or other limiting devices that prevent windows from opening fully, the post-fire smoke purge system is still mandatory.
Post-fire smoke purge plays a very important role in building interiors, reestablishing suitable conditions for occupancy as soon as possible after a fire is controlled. Real estate developers can seek professional engineering services to help them decide between stair ventilation and corridor ventilation in new constructions. No option can be considered better than the other, since each has advantages and disadvantages.
It is important to note that the requirements of other NYC codes also apply for post-fire smoke purge systems, including the NYC Fire Code and Mechanical Code. Getting professional assistance is the most effective way to ensure a new project is code-compliant.
At Nearby EngineersNew York Engineers , we search for simple, eloquent solutions to complex problems. We minimize construction costs by eliminating the extraneous and focusing on the overall efficiency for the most streamlined designs.