Fire Rated Ductwork
When and why fire rated ductwork is required
Limiting the spread of smoke and fire throughout a building, using effective zones and ventilation duct, is one of the most important factors to consider when designing and constructing a building. Under normal circumstances, this zoning is achieved by the use of fire dampers or fire/smoke dampers within the duct systems. However, under certain applications, the use of fire dampers is inappropriate. In this case, the ducts themselves must be fire rated, providing the same level of fire rating afforded by the wall or floor of the zone.
Some systems, which would not be fitted with fire dampers under normal circumstances and, therefore, must be fire rated, are:
- Smoke Exhaust Ductwork Systems
- Grease Duct Systems
- Exit/Escape Route Duct Systems
- Pressurization Ductwork Systems
- Basement Smoke Exhaust Systems
- Non-commercial kitchen exhausts
- Dryer exhausts
- Fire Smoke Emergency Exhausts
- Dual Ventilation/Smoke Exhaust Systems
Some systems, which can have fire dampers, but would also benefit from being fire-rated are:
- Fume Hood Exhaust Systems
- Combustion air supply and exhausts
- High-rise, toilet and kitchen exhaust systems
Exhaust systems serving commercial kitchen hoods are required, as per NFPA 96, to be separate and independent of any other systems because of the polluted nature of the extracted air. Fire dampers should not be installed in any exhaust duct systems serving kitchens. Any ducts that penetrate fire resistive barriers should be fire rated to the same level of fire separation as the wall or floor through which they pass. The fire rating should be for both Type B exposure (fire on the inside) and engulfment - Type A exposure (fire on the outside). All penetrations are required to be fire stopped.
Grease ducts pose a particular hazard in that they contain deposits of accumulated grease on the internal surfaces. Fire in an adjacent zone through which the duct passes could ignite the internal deposits and cause a flashback to the served kitchen hood. It is important, therefore, that all grease ducts be insulated to prevent this. To minimise the build up of grease, fire rated access doors shall be installed at maximum 10 feet centres to allow access for cleaning.
Grease duct installations shall be evaluated, tested and listed to the requirements of the IBC, IMC, and UMC and these require compliance with ASTM E-2336, UL 1978 or UL 2221. FLAMEBAR BW11 complies with ASTM E-2336, UL 1978, and NFPA 96.
It is a requirement of the International Mechanical Code that any duct crossing an exit/escape corridor or lobby must be fire rated for stability, integrity, and insulation, with the same rating as the compartment through which it passes. It should be noted that, with FLAMEBAR BW11, only one fire damper is required if either side of the exit corridor is constructed with non-fire rated ductwork.
Pressurization is a method of restricting the penetration of smoke into exit/escape routes, protected stairwells, corridors, and fire fighting shafts for deep basement by maintaining pressure differential. Smoke in these areas would jeopardise escape in the event of a fire. NFPA 90A requires that pressurization ducts should be enclosed in a two-hour fire rated enclosure. The FLAMEBAR BW11 duct is classified by UL for up to four hours stability, integrity, and insulation without any additional enclosure.
NFPA 90A requires that non-commercial kitchen riser ducts on high-rise apartments passing from floor to floor up through the building be protected in a two-hour fire rated enclosure. Since the FLAMEBAR BW11 duct system is a two-hour rated assembly, the need for a two-hour enclosure is eliminated. However, as defined in NFPA 90A, all branches serving each floor should be fitted with a fire damper, fire/smoke damper, or a SMACNA type (sub-duct).
NFPA 90A requires that all dryer extracts rising through a building and passing from one fire zone to the next be enclosed in a two-hour rated duct/shaft. Since the FLAMEBAR BW11 Duct System is a two-hour rated assembly, the need for a two-hour enclosure can be eliminated. The Building Code further stipulates that the internal face of the ducts should be free from screws and protrusions that could cause lint to be trapped.
If the duct system is intended to be used to remove hot smoke in an emergency, ISO 6944 (1985) requires that the duct maintain 75% of its original free area. If a duct crosses from the fire compartment to an adjacent fire zone, it must have the same fire resistance as the fire barrier it crosses. Ducts used as smoke extractors should be rated for fire exposure Type B (fire on the inside).
These serve as normal ventilation systems but, in an emergency, are converted to provide smoke extraction in the event of a fire. Since the FLAMEBAR BW11 duct is already fire rated, no additional enclosures are necessary. By combining the two applications into one system, significant cost savings can be affected.
NFPA 90A requires that toilet extract riser ducts on high-rise apartments passing from floor to floor up through the building be protected in a two-hour fire rated enclosure. Since the FLAMEBAR BW11 duct system is a two-hour rated assembly, the need for a two-hour enclosure is eliminated. However, as defined in NFPA 90A, all branches serving each floor shall be fitted with a fire damper or a SMACNA sub-duct.