Integration: HVAC fans and smoke control
There is no real difference between ductwork in the distribution system used for HVAC and smoke control systems. The only changes are that the ductwork is required to be supported by fire-resistance rated structural elements using noncombustible supports. There is no specific requirement as to the type of material to be used, other than what is required for HVAC systems. Ducts can be made from sheet metal or can be made of drywall.
Duct materials and joints are required by the IBC to be capable of withstanding the probable temperatures and pressures to which they will be exposed. They are required to be leak tested to 1.5 times the maximum design pressure. The designer needs to document the design pressure of the duct, which is often different than the fan static pressure. The measured leakage cannot exceed 5% of the design flow. While not specifically outlined in the code, leak testing is typically done only for exhaust systems that traverse other smoke zones to confirm they will not leak significant amounts of smoke when exhausting the fire zone. Ducts that are contained only within the zone or supply air systems do not have the potential to convey smoke outside of the initial zone. Therefore, it is typical to see the designer identify those ducts requiring leak testing and not all ducts. It would be reasonable to expect vertical ducts traversing multiple floors to be leak tested, but not the portion of the duct that is isolated to the zone.
Another issue that has been encountered in the use of HVAC systems for smoke control is the use of ducts made of drywall materials. Leakage rates for HVAC systems may not be impacted for climate control the same as they are for smoke control. Experience has shown that when using drywall ducts for smoke control systems, it is difficult at best to meet the leakage testing requirements. Often metal ductwork has had to be installed to replace the drywall ducts to meet the leak testing. Therefore, metal ductwork should be used wherever possible to prevent impact by leak testing during commissioning of the system.
Additional considerations should be taken when determining the location of exhaust/return fan discharge outlets and their associated fans. Fans used for exhaust in a smoke control or smoke removal system should not discharge at locations where the potential exists for reintroducing smoke into the building or any adjacent buildings.
Smoke control mode, as defined by NFPA 92, is a predefined operational configuration of a system or device for the purpose of smoke control. This is accomplished by a set of predefined sequences programmed into the smoke control system. The smoke control system can be either a subset of the building’s fire alarm system, or a combination of the building’s automation and fire alarm system. When HVAC systems are being used, some form of building automation system (BAS) is commonly part of the overall smoke control system.
A smoke control system requires input from the field to determine when to configure the system. This is done primarily through the fire alarm system’s smoke detectors or monitoring of the building’s automatic sprinkler system. When the fire alarm system detects one of these conditions, a signal is processed and the smoke control system is configured for its predetermined sequence for that zone in the building. Depending upon the control system concept employed, either the fire alarm system drives all sequences overriding the BAS or a handoff of signals occurs allowing the BAS to control all or a portion of the system. In a typical high-rise building, it is usually a combination of the BAS and fire alarm system controlling equipment.
It is common to see both BAS and fire alarm systems control the overall smoke control system. While it is also common to see just the fire alarm system, this can duplicate controls and wiring and drive costs. If the BAS is suited for smoke control functions (in the United States, UUKL listed by UL for smoke control purposes), then it is acceptable to have both systems control fans and dampers appropriately. In high-rise office buildings where HVAC systems are used for both climate and smoke control, the fire alarm system can hand off a signal to the HVAC/BAS to configure dampers and fans already being controlled for climate control purposes. Other fans, such as stairwell pressurization fans, do not have HVAC controls and can be controlled directly by the fire alarm system.
The designers of the smoke control and HVAC systems must collaborate to determine the most cost-effective and reliable source for controlling the system. If the designers are one and the same, consideration should be given to how to achieve the proper control functions using reliable equipment already in place. This not only applies to the field circuits and controls but also to graphic interface panels. It is not practical to have two separate graphic control panels for one overall smoke control system. When using two control systems, the user interface needs to be considered to allow for efficient control and overrides of the system in an emergency condition.
Overall, it is common to see HVAC systems employed for smoke control purposes. It can save money and can provide for reliable operation. When considering the use of dual-purpose equipment, such as HVAC systems, take into account the impact the smoke control operation and code requirements have on HVAC functions and whether the HVAC system can function as intended for emergency conditions.
Allyn J. Vaughn is CTO/president at jba consulting engineers. He has been in Las Vegas for more than 15 years providing fire protection system design and code consulting services, including design and commissioning of smoke control systems. Stephen Haines is project engineer with jba consulting engineers. He has more than 5 years of experience designing HVAC systems for smoke control in various high-rise buildings in Nevada, Arizona, California, and Macau, China.