FFPC 101:9.2 Heating, Ventilating, and Air-Conditioning
9.2.1 Air-Conditioning, Heating, Ventilating Ductwork, and Related Equipment.
Air-conditioning, heating, ventilating ductwork, and related equipment shall be in accordance with NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, or NFPA 90B, Standard for the Installation of Warm Air Heating and Air-Conditioning Systems, as applicable, unless such installations are approved existing installations, which shall be permitted to be continued in service.
Commentary from the 2012 Life Safety Code Handbook: For the proper installation of HVAC systems, 9.2.1 refers the Code user to NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems. For occupancies with small overall volumes, such as one-and two-family dwellings, the Code refers the user to NFPA 90B, Standard for the Installation of Warm Air Heating and Air-Conditioning Systems.5
For example, NFPA 90A addresses fire damper requirements for both ductwork and air-transfer grilles that penetrate fire resistance–rated barriers. NFPA 90A also prohibits means of egress corridors in health care, detention and correctional, and residential occupancies from being used as a portion of a supply-, return-, or exhaust-air system serving adjoining areas. Exhibit 9.1 identifies some of the areas where fire dampers and smoke dampers would be required by NFPA 90A.
FFPC 101:126.96.36.199.2* Smoke detectors used solely for closing dampers or heating, ventilating, and air-conditioning system shutdown shall not be required to activate the building evacuation alarm, provided that the power supply and installation wiring to the detectors are monitored by the building fire alarm system, and the activation of the detectors initiates a supervisory signal at a constantly attended location. [FFPC 1:188.8.131.52.10.2.3*]
A.184.108.40.206.2 The concept addressed is that detectors used for releasing service, such as door or damper closing and fan shutdown, are not required to sound the building alarm. [FFPC 1:A.220.127.116.11.10.2.3]
FFPC 101:9.6.5 Fire Safety Functions.
18.104.22.168 Fire safety functions shall be installed in accordance with the requirements of NFPA 72, National Fire Alarm and Signaling Code. [FFPC 1:22.214.171.124.12.1]
126.96.36.199 Where required by another section of this Code, the following functions shall be actuated:
(1) Release of hold-open devices for doors or other opening protectives
(2) Stairwell or elevator shaft pressurization
(3) Smoke management or smoke control systems
(4) Unlocking of doors
(5) Elevator recall and shutdown
(6) HVAC shutdown
Commentary from the 2012 Life Safety Code Handbook: Requirements for automatic shutdown of heating, ventilating, and air-conditioning (HVAC) equipment upon detection of smoke are contained in NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems. Where required, automatic HVAC shutdown can be accomplished by duct smoke detectors that are not part of a building fire alarm system. Alternatively, HVAC systems can be arranged to automatically shut down upon detection of smoke by open area smoke detectors that are connected to the building fire alarm system in accordance with NFPA 72, National Fire Alarm and Signaling Code. See NFPA 72 and NFPA 90A for additional details.
In addition to the items listed in 188.8.131.52(1) through (6), fire alarm systems can be used to perform other fire safety control functions. For example, new special amusement buildings are addressed in 12.4.7. Those that operate in reduced lighting levels (e.g., a haunted house amusement) must, upon actuation of the required automatic smoke detection system or the required automatic sprinkler system, increase illumination in the means of egress to at least the minimum level required by Section 7.8. The requirements applicable to special amusement buildings augment the concept of using the alarm system to initiate an emergency control function by mandating that conflicting or confusing sounds and visual effects stop upon actuation of the required automatic smoke detection system or the required automatic sprinkler system. Thus, in a haunted house amusement, all audible and visual special effects would cease to operate upon alarm, so as not to confuse the patrons. A fire alarm horn or strobe might not be recognized if forced to compete with the background special effects common to such an occupancy.
MECHANICAL CODE: SECTION 606 SMOKE DETECTION SYSTEMS CONTROL
606.1 Controls required.
Air distribution systems shall be equipped with smoke detectors listed and labeled for installation in air distribution systems, as required by this section. Duct smoke detectors shall comply with UL 268A. Other smoke detectors shall comply with UL 268.
606.2 Where required.
Smoke detectors shall be installed where indicated in Sections 606.2.1 through 606.2.3.
Exception: Smoke detectors shall not be required where air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors and ceilings of the room or space in which the smoke is generated.
606.2.1 Return air systems.
Smoke detectors shall be installed in return air systems with a design capacity greater than 2,000 cfm (0.9 m3/s), in the return air duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment and appliances.
Exception: Smoke detectors are not required in the return air system where all portions of the building served by the air distribution system are protected by area smoke detectors connected to a fire alarm system in accordance with the Florida Fire Prevention Code. The area smoke detection system shall comply with Section 606.4.
606.2.2 Common supply and return air
Where multiple air-handling systems share common supply or return air ducts or plenums with a combined design capacity greater than 2,000 cfm (0.9 m3/s), the return air system shall be provided with smoke detectors in accordance with Section 606.2.1.
Exception: Individual smoke detectors shall not be required for each fan-powered terminal unit, provided that such units do not have an individual design capacity greater than 2,000 cfm (0.9 m3/s) and will be shut down by activation of one of the following:
1. Smoke detectors required by Sections 606.2.1 and 606.2.3.
2. An approved area smoke detector system located in the return air plenum serving such units.
area smoke detector system as prescribed in the exception to Section
In all cases, the smoke detectors shall comply with Sections 606.4 and 606.4.1.
606.2.3 Return air risers.
Where return air risers serve two or more stories and serve any portion of a return air system having a design capacity greater than 15,000 cfm (7.1 m3/s), smoke detectors shall be installed at each story. Such smoke detectors shall be located upstream of the connection between the return air riser and any air ducts or plenums.
Smoke detectors required by this section shall be installed in accordance with NFPA 72. The required smoke detectors shall be installed to monitor the entire airflow conveyed by the system including return air and exhaust or relief air. Access shall be provided to smoke detectors for inspection and maintenance.
606.4 Controls operation.
Upon activation, the smoke detectors shall shut down all operational capabilities of the air distribution system in accordance with the listing and labeling of appliances used in the system. Air distribution systems that are part of a smoke control system shall switch to the smoke control mode upon activation of a detector.
The duct smoke detectors shall be connected to a fire alarm system where a fire alarm system is required by the Florida Fire Prevention Code. The actuation of a duct smoke detector shall activate a visible and audible supervisory signal at a constantly attended location.
1. The supervisory signal at a constantly attended location is not required where the duct smoke detector activates the building’s alarm-indicating appliances.
2. In occupancies not required to be equipped with a fire alarm system, actuation of a smoke detector shall activate a visible and audible signal in an approved location. Duct smoke detector trouble conditions shall activate a visible or audible signal in an approved location and shall be identified as air duct detector trouble.
NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems 2012 Edition
6.4* Smoke Detection for Automatic Control.
A. 6.4 The use of smoke detectors in relationship to HVAC systems and high air movement areas and the detailed regarding their optimum installation are covered in Section 5.7 of NFPA 72, National Fire Alarm Code.
Protection provided by the installation of smoke detectors and related requirements is intended to prevent the distribution of smoke through the supply air duct system and, preferably, to exhaust a significant quantity of smoke to the outside. Neither function, however, guarantees either the early detection of fire or the detection of smoke concentrations prior to dangerous smoke conditions where smoke movement is other than through the supply air system.
Where smoke-control protection for a facility is determined to be needed, see NFPA 92A, Standard for Smoke-Control Systems Utilizing Barrier and Pressure Differences.
6.4.1 Testing. All automatic shutdown devices shall be tested at least annually.
A.6.4.2 The summation of capacities of individual supply-air fans should be made where such fans are connected to a common supply air duct system (i.e., all fans connected to a common air duct supply system should be considered as constituting a single system with respect to the applicability of the Chapter 6 provisions that dependent on system capacity).
184.108.40.206 Smoke detectors listed for use in air distribution systems shall be located as follows:
(1) Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)
(2) At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 7080 L/sec (15,000 ft3/min) and serving more than one story
220.127.116.11 Return system smoke detectors shall not be required where the entire space served by the air distribution system is protected by a system of area smoke detectors.
18.104.22.168 Smoke detectors shall not be required for fan units whose sole function is to remove air from the inside of the building to the outside of the building.
A.6.4.3 Where automatic water sprinklers are provided to coordinate with the HVAC zones, their water flow switches should imitate devices for the function described in Chapter 6.
Sprinklers are often tested weekly. Where it is desirable to prevent the accompanying automatic shutdown of fan system(s) reference 6.4.3, a means can be permitted to be used to avoid such shutdown temporarily, provided one of the following occurs:
(1) A trouble signal is sustained in the sprinkle supervisory system until the automatic shutdown provision is restored.
(2) The automatic shutdown provision is restored at the end of the time period necessary to test the sprinkler system, its alarms, and related elements.
22.214.171.124 Smoke detectors provided as required by 6.4.2 shall automatically stop their respective fan(s) on detecting the presence of smoke.
126.96.36.199 Where the return air fan is functioning as part of an engineered smoke-control system and a different mode is required, the smoke detectors shall not be required to automatically stop their respective fans.
188.8.131.52 Smoke detectors shall be installed, tested, and maintained in accordance with NFPA 72, National Fire Alarm and Signaling Code.
184.108.40.206 In addition to the requirements of 6.4.3, where an approved fire alarm system is installed in a building, the smoke detectors required by the provisions of Section 6.4 shall be connected to the fire alarm system in accordance with the requirements of NFPA 72, National Fire Alarm and Signaling Code.
220.127.116.11.1 Smoke detectors used solely for closing dampers or for heating, ventilating, and air-conditioning system shutdown shall not be required to activate the building evacuation alarm.
18.104.22.168 Where smoke detectors required by Section 6.4 are installed in a building not equipped with an approved fire alarm system as specified by 22.214.171.124, the following shall occur:
(1) Smoke detector activation required by Section 6.4 shall cause a visual signal and an audible signal in a normally occupied area.
(2) Smoke detector trouble conditions shall be indicated visually or audibly in a normally occupied area and shall be identified as air duct detector trouble.
126.96.36.199 Smoke detectors powered separately from the fire alarm system for the sole function of stopping fans shall not require standby power.
NFPA 72, National Fire Alarm and Signaling Code, 2010 Edition
17.7.4 Heating, Ventilating, and Air-Conditioning (HVAC).
188.8.131.52* In spaces served by air-handling systems, detectors shall not be located where airflow prevents operation of the detectors.
A.184.108.40.206 Detectors should not be located in a direct airflow or closer than 36 in. (910 mm) from an air supply diffuser or return air opening. Supply or return sources larger than those commonly found in residential and small commercial establishments can require greater clearance to smoke detectors. Similarly, smoke detectors should be located farther away from high velocity air supplies.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: For years, the rule in 220.127.116.11 had been applied only to air supplies. In research conducted under the International Fire Detection Research Project, managed by the Fire Protection Research Foundation, the computer modeling conducted by the National Institute of Standards and Technology (NIST) identified situations where areas of nonactuation extended almost 11 ft (3.4 m) from some supply diffusers. In addition, the research showed that a smoke dilution effect occurred near air returns. An air return pulls air up from levels in the room that are beneath the ceiling jet, which has the effect of diluting smoke concentration near the air return grille. Consequently, the designer should arrange the detection so that detectors are not adjacent to either air supplies or air returns.
Situations may exist where even a 3 ft (0.9 m) separation is not adequate. This situation would depend on the air velocity (supply air and return air) and the throw characteristics of the supply diffuser and diffuser size. Unfortunately, because the research did not address wide variations in HVAC flow rates, the minimum distance between a detector and the HVAC system supply or return recommended in A.18.104.22.168 might not be valid in all cases. Where in doubt, airflow in the vicinity of the detector should be mapped with a velometer or anemometer. Certainly, the ambient airflow at the detector location should be only a fraction of that used in the UL 268 smoke box of 30 ft/min (0.152 m/sec).
22.214.171.124 Detectors installed in plenums shall comply with 126.96.36.199.1 and 188.8.131.52.2.
184.108.40.206.1 In under-floor spaces and above-ceiling spaces that are used as HVAC plenums, detectors shall be listed for the anticipated environment as required by 220.127.116.11. Detector spacings and locations shall be selected on the basis of anticipated airflow patterns and fire type.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: To cool a room to 70°F (21°C), the introduction of extremely frigid air into the room may be necessary. Conversely, heating a room sometimes requires introducing extremely hot air into a room. Consequently, HVAC plenums usually have ambient conditions that are far more extreme than the spaces they support.
Smoke detectors are electronic sensors. Ambient temperature, the relative humidity, and, especially in the case of spot-type ionization detectors, the velocity of the air around the detector all affect detector operation. Not all smoke detectors are listed for the range of conditions found in HVAC plenums or in under-floor or above-ceiling spaces. It is the designer’s responsibility to verify that the detector is listed for use in the range of environmental conditions that will be encountered where it is to be installed. See also 18.104.22.168 and A.22.214.171.124.
126.96.36.199.2* Detectors placed in environmental air ducts or plenums shall not be used as a substitute for open area detectors. Where detectors are used for the control of smoke spread, the requirements of 17.7.5 shall apply. Where open area protection is required, 17.7.3 shall apply.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: In most buildings, there are times when the HVAC system is not moving significant quantities of air from the compartments it serves. This is typical of variable air volume (VAV) systems. Consequently, the fire detection system cannot be designed to rely on the HVAC system operation for the transport of smoke to smoke detectors.
A.188.8.131.52.2 Smoke might not be drawn into the duct or plenums when the ventilating system is shut down. Furthermore, when the ventilating system is operating, the detector(s) can be less responsive to a fire condition in the room of fire origin due to dilution by clean air.
17.7.5* Smoke Detectors for Control of Smoke Spread.
A.17.7.5 Refer to NFPA 101, Life Safety Code, for the definition of smoke compartment; NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, for the definition of duct systems; and NFPA 92A, Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences, for the definition of smoke zone.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Between 1960 and 1971, several fires in high-rise buildings demonstrated the difficulty of trying to evacuate an entire building. Not only did occupants incur injuries during the evacuation, but also the means of egress often became untenable due to heavy smoke concentrations.
As improved building codes resulted in structures that could maintain their integrity in spite of the complete combustion of the interior fire load through passive fire-resistive construction and compartmentation, defending occupants in place became a viable option. Strategies for establishing smoke compartments and areas of refuge and for managing the flow of smoke by directing it away from the occupants were developed. Experiences with high-rise fires indicate that the proactive control of smoke with either automatic smoke detectors and HVAC systems or engineered smoke control systems is a viable strategy for occupant protection in high-rise buildings.
Subsection 17.7.5 does not require the installation of smoke detectors for smoke control. The purpose of 17.7.5 is to describe the performance and installation requirements for smoke detectors being used for smoke control, as required by some other code or standard.
184.108.40.206* Classifications. Smoke detectors installed and used to prevent smoke spread by initiating control of fans, dampers, doors, and other equipment shall be classified in the following manner:
(1) Area detectors that are installed in the related smoke compartments
(2) Detectors that are installed in the air duct systems
(3) Video image smoke detection that is installed in related smoke compartments
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Either dedicated detectors installed in the HVAC system or area detectors can be used to control smoke spread. With modern addressable/analog detection technology, individual ceiling-mounted spot-type detectors produce discrete alarm signal codes that are logged by the fire alarm control unit. This technology permits the use of area detection without the incremental cost of large numbers of detector relays, as was the case decades ago. Both projected beam smoke detectors and video image smoke detectors are also used as area detection and can be used as an input signal for the control of the HVAC system serving the related smoke compartments.
A.220.127.116.11 Smoke detectors located in an open area(s) should be used rather than duct-type detectors because of the dilution effect in air ducts. Active smoke management systems installed in accordance with NFPA 92A, Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences, or NFPA 92B, Standard for Smoke Management Systems in Malls, Atria, and Large Spaces, should be controlled by total coverage open area detection.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Paragraph 18.104.22.168 identifies all the spaces that must have smoke detectors if total coverage is to be achieved.
A.22.214.171.124 Dilution of smoke-laden air by clean air from other parts of the building or dilution by outside air intakes can allow high densities of smoke in a single room with no appreciable smoke in the air duct at the detector location. Smoke might not be drawn from open areas if air-conditioning systems or ventilating systems are shut down.
126.96.36.199.1 Detectors that are installed in the air duct system in accordance with 188.8.131.52(2) shall not be used as a substitute for open area protection.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: All too often, uninformed designers attempt to use air duct–type smoke detectors to provide open area protection. This strategy does not address the potential for a fire during those times when the HVAC system is not running, nor does it address the delay in detection due to smoke dilution. Paragraph 184.108.40.206.1 specifically prohibits the use of duct smoke detection in lieu of area detection installed pursuant to Section 17.7.
220.127.116.11.2 Where open area protection is required, 17.7.3 shall apply.
A.18.104.22.168 Smoke detectors can be applied in order to initiate control of smoke spread for the following purposes:
(1) Prevention of the recirculation of dangerous quantities of smoke within a building
(2) Selective operation of equipment to exhaust smoke from a building
(3) Selective operation of equipment to pressurize smoke compartments
(4) Operation of doors and dampers to close the openings in smoke compartments
22.214.171.124.1 To prevent the recirculation of dangerous quantities of smoke, a detector approved for air duct use shall be installed on the supply side of air-handling systems as required by NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, and 126.96.36.199.2.1.
188.8.131.52.2 If smoke detectors are used to initiate selectively the operation of equipment to control smoke spread, the requirements of 184.108.40.206.2.2 shall apply.
220.127.116.11.3 If detectors are used to initiate the operation of smoke doors, the requirements of 18.104.22.168 shall apply.
22.214.171.124.4 If duct detectors are used to initiate the operation of smoke dampers within ducts, the requirements of 126.96.36.199 shall apply.
188.8.131.52.1 Area Smoke Detectors Within Smoke Compartments. Area smoke detectors within smoke compartments shall be permitted to be used to control the spread of smoke by initiating operation of doors, dampers, and other equipment.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Paragraph 184.108.40.206.1 allows area detectors to serve the additional purpose of providing signals to initiate the control of the spread of smoke. Although this approach might not have been very practical when the only available technology was conventional detection on an initiating device circuit, now addressable/analog detectors, whose principal function is area protection, can be used effectively to provide signals that are then used to control smoke spread. Existing detectors can perform double duty through the programming of the fire alarm control unit. When area smoke detectors are used, smoke detectors are needed where they can identify the presence of smoke at a particular location or the movement of smoke past a particular location. The locations for area smoke detectors are a function of building geometry, anticipated fire locations, and intended goals of smoke control functions.
Except where used as permitted in 220.127.116.11.2.2(B), complete area smoke detection is not necessary to provide for such control features. Specific locations are often identified for specific fire scenarios. For example, smoke detectors are often placed at the perimeter of an atrium to detect smoke movement into the atrium space from a corridor that opens into the atrium. Another example is the use of smoke detectors to release smoke doors only as their associated smoke detector is actuated, thus avoiding premature release of all other doors. Selective door release is sometimes chosen to prevent the premature release of doors needed to facilitate rapid evacuation.
Paragraph 18.104.22.168.1 also allows complete area coverage to be used for the control of smoke spread. In this case, when a compartment detector actuates in the smoke compartment, it signals the fire alarm control unit, which, in turn, signals the HVAC control system or smoke door release system. The HVAC controller operates or controls fans and dampers to prevent the introduction of smoke into other smoke compartments and to vent the smoke from the fire compartment, facilitating occupant egress. The smoke door release system either closes all doors in the building or all doors in the smoke zone.
22.214.171.124.2* Smoke Detection for the Air Duct System.
A.126.96.36.199.2 Smoke detectors are designed to sense the presence of particles of combustion, but depending on the sensing technology and other design factors, different detectors respond to different types of particles. Detectors based on ionization detection technology are most responsive to smaller, invisible sub-micron sized particles. Detectors based on photoelectric technology, by contrast, are most responsive to larger visible particles.
It is generally accepted that particle size distribution varies from sub-micron diameter particles predominant in the proximity of the flame of a flaming fire to particles one or more orders of magnitude larger, which are characteristic of smoke from a smoldering fire. The actual particle size distribution depends on a host of other variables including the fuel and its physical make-up, the availability of oxygen including air supply and fire–gas discharge, and other ambient conditions, especially humidity. Moreover, the particle size distribution is not constant, but as the fire gases cool, the sub-micron particles agglomerate and the very large ones precipitate. In other words, as smoke travels away from the fire source, the particle size distribution shows a relative decrease in smaller particles. Water vapor, which is abundantly present in most fires, when cooled sufficiently will condense to form fog particles — an effect frequently seen above tall chimneys. Because water condensation is basically clear in color, when it is mixed with other smoke particles, it can be expected to lighten the color of the mixture.
In almost every fire scenario in an air-handling system, the point of detection will be some distance from the fire source; therefore, the smoke will be cooler and more visible because of the growth of sub-micron particles into larger particles due to agglomeration and recombination. For these reasons, photoelectric detection technology has advantages over ionization detection technology in air duct system applications.
188.8.131.52.2.1 Supply Air System. Where the detection of smoke in the supply air system is required by other NFPA standards, a detector(s) listed for the air velocity present and that is located in the supply air duct downstream of both the fan and the filters shall be installed.
Exception: Additional smoke detectors shall not be required to be installed in ducts where the air duct system passes through other smoke compartments not served by the duct.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: The NFPA standards relevant to 184.108.40.206.2.1 are NFPA 90A; NFPA 92A, Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences; and NFPA 101. The purpose of supply-side smoke detection is the sensing of smoke that might be contaminating the area served by the duct but not as a result of a fire in that area. The smoke might be coming from another area via return air ducts, from outside via fresh air mixing ducts, or from a fire within the duct (such as in a filter or fan belt). If the source of the smoke is from outside or from within the duct, a fire alarm response for area detection within the space would not normally be expected to produce the most appropriate set of responses.
Different airflow management programs are required for supply-side smoke inflow as opposed to smoke generated within the compartment. Furthermore, compartment area detection cannot be relied on to respond to a supply duct smoke inflow, because of the expected dilution of smoke-laden air with fresh air as it enters the smoke compartment where the area detection is installed. This expected condition necessitates the use of detectors downstream of the fan and filters in the supply air duct.
The exception to 220.127.116.11.2.1 is based on the fire resistance of HVAC ducts and the unlikelihood of smoke escaping from the HVAC duct into a compartment not served by the duct. Refer to the following excerpt from NFPA 90A for supply and return air smoke detection requirements.
6.4.2* Location. [90A:6.4.2]
18.104.22.168 Smoke detectors listed for use in air distribution systems shall be located as follows:
(1) Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)
(2) At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 7080 L/sec (15,000 ft3/min) and serving more than one story [90A:22.214.171.124]
126.96.36.199 Return system smoke detectors shall not be required where the entire space served by the air distribution system is protected by a system of area smoke detectors. [90A:188.8.131.52]
184.108.40.206 Smoke detectors shall not be required for fan units whose sole function is to remove air from the inside of the building to the outside of the building. [90A:220.127.116.11]
18.104.22.168.2.2* Return Air System. Unless otherwise modified by 22.214.171.124.2.2(A) or 126.96.36.199.2.2(B), if the detection of smoke in the return air system is required by other NFPA standards, a detector(s) listed for the air velocity present shall be located where the air leaves each smoke compartment, or in the duct system before the air enters the return air system common to more than one smoke compartment.
(A) Additional smoke detectors shall not be required to be installed in ducts where the air duct system passes through other smoke compartments not served by the duct.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Paragraph 188.8.131.52.2.2(A) is based on the same reasoning used in the exception to 184.108.40.206.2.1. With reference to Figure A.220.127.116.11.2.2(c), the top duct does not need additional detectors and/or dampers where it passes through either the center compartment or the right compartment.
(B) Where total coverage smoke detection is installed in all areas of the smoke compartment served by the return air system, installation of air duct detectors in the return air system shall not be required, provided that their function is accomplished by the design of the area detection system.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: The key phrase in 18.104.22.168.2.2(B) is “provided that their function is accomplished by the design of the area detection system.” When an engineering analysis shows that the area smoke detection addresses all the smoke ingress paths from the compartment into the return air duct, this allowance is operative. Naturally, the analysis must be fully documented and made part of the permanent fire alarm system design file.
A.22.214.171.124.2.2 Detectors listed for the air velocity present can be permitted to be installed at the opening where the return air enters the common return air system. The detectors should be installed up to 12 in. (300 mm) in front of or behind the opening and spaced according to the following opening dimensions [see Figure A.126.96.36.199.2.2(a) through Figure A.188.8.131.52.2.2(c)]:
(a) Up to 36 in. (910 mm) — One detector centered in opening
(b) Up to 72 in. (1.83 m) — Two detectors located at the one-quarter points of the opening
(c) Over 72 in. (1.83 m) — One additional detector for each full 24 in. (610 mm) of Opening
(2) Depth. The number and spacing of the detector(s) in the depth (vertical) of the opening should be the same as those given for the width (horizontal) in A.184.108.40.206.2.2(1).
(3) Orientation. Detectors should be oriented in the most favorable position for smoke entry with respect to the direction of airflow. The path of a projected beam–type detector across the return air openings should be considered equivalent in coverage to a row of individual detectors.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: The objective of HVAC system return detection is to prevent the recirculation of smoke-laden air to other, smoke-free portions of the building via the HVAC system. While use of complete area detection is preferable because it provides the earliest possible response, the use of return duct detection is permitted and most often used.
If duct detection is used for control of smoke spread, detectors must be installed only where the return air duct leaves the smoke compartment or before the duct joins a return air plenum serving more than one smoke compartment. These locations are intended to minimize the effects of smoke dilution.
The specific detector location criteria outlined in A.220.127.116.11.2.2 are intended to achieve a representative sample of the air flowing into the system. The HVAC system return will draw air from a portion of the room volume based on its location. Ceiling returns pull fresh air up from lower elevations in the room, through the ceiling jet, diluting the smoke. Wall-mounted returns also tend to draw in air from a range of elevations in the room, reducing the relative smoke concentration. Consequently, dilution is almost always present and almost always delays response. Therefore, dilution is one of the reasons that duct-type smoke detection will be slower than spot detection in the area of the fire.
18.104.22.168 Location and Installation of Detectors in Air Duct Systems.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Sampling tubes provide a flow of air through the detector enclosure due to a pressure differential that results from the flow of air across the tubes. Small errors in the orientation of the sampling tubes can reduce the pressure differential, rendering them ineffective in drawing air into the detector enclosure, especially at low air velocities in variable air volume (VAV) HVAC systems.
For sampling tubes to take a representative sample of the air passing through the duct, they must be fabricated and installed in a manner consistent with their listing. The pressure differential between the inflow and outflow tubes is usually measured with either a manometer or pressure gauges. (See Exhibit 14.11.) If the flow of air through the sampling tube and the detector enclosure assembly cannot be verified, as required by 22.214.171.124, there is no basis to presume that the air within the duct is being sampled by the detector. Prudent practice dictates that the pressure differential be measured at the lowest air velocity anticipated for the duct where the detector is located in a VAV HVAC system.
Finally, duct-type smoke detectors usually consist of a standard production smoke detector and a specially designed enclosure equipped with a smoke detector mounting base and sampling tube fittings already installed. However, not all detectors are listed for use in a duct smoke detector enclosure that uses sampling tubes. Care should be taken to make certain that the detector is listed for use in the duct smoke detector housing as an assembly.
126.96.36.199.1 Detectors shall be listed for the purpose for which they are being used.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: The listing of the detector stipulates the range of air velocities over which it can operate, as well as the temperature and the relative humidity range. These last two criteria are particularly important where a general purpose detector is being installed in a duct detector housing. Often HVAC system fans and ducts are located in penthouses and mechanical rooms, where comfort heating and cooling are not provided. Consequently, it is possible that a smoke detector will be inadvertently installed where the ambient conditions exceed its design range. The location of the duct detector must be maintained within the operating range of the detector used.
Support of the detector by the conduit or raceway containing wiring conductors is not permitted by NFPA 70 unless the box is specifically listed for the purpose and installed in accordance with the listing.
188.8.131.52.2* Air duct detectors shall be installed in such a way as to obtain a representative sample of the airstream. This installation shall be permitted to be achieved by any of the following methods:
(1) Rigid mounting within the duct
(2) Rigid mounting to the wall of the duct with the sensing element protruding into the duct
(3) Installation outside the duct with rigidly mounted sampling tubes protruding into the duct
(4) Installation through the duct with projected light beam
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: The flow of air through a duct is not necessarily uniform. Bends and changes in cross-sectional area and cross-sectional shape of the duct produce regions of reduced flow velocity and, hence, reduced flow volume. The flow in a duct can also become divided into layers depending on differing temperatures, resulting in smoke being concentrated in a portion of the duct cross-section and not uniformly dispersed across the duct area. The options in 184.108.40.206.2(1) and 220.127.116.11.2(2) are often most appropriate for smaller ducts or where an engineering analysis shows that smoke concentrations will be even across the duct cross-section and that laminar flow is not going to produce a non-uniform smoke concentration. Option (3) is more suited to larger ducts. The use of sampling tubes enables the duct detector to sample the air across the entire duct cross-section rather than a small portion of it. The designer should consult the manufacturer’s technical bulletin for installation limitations.
See Exhibits 17.31 and 17.32 for examples of typical duct-type smoke detectors.
A.18.104.22.168.2 Where duct detectors are used to initiate the operation of smoke dampers, they should be located so that the detector is between the last inlet or outlet upstream of the damper and the first inlet or outlet downstream of the damper.
In order to obtain a representative sample, stratification and dead air space should be avoided. Such conditions could be caused by return duct openings, sharp turns, or connections, as well as by long, uninterrupted straight runs.
In return air systems, the requirements of 22.214.171.124.2.2 take precedence over these considerations. [See Figure A.126.96.36.199.2(a) and Figure A.188.8.131.52.2(b).]
Usually, it is necessary to manage smoke flow in buildings. Duct smoke detectors are used to shut down HVAC systems or initiate smoke management.
Filters have a serious effect on the performance of duct smoke detectors. The location of the detector relative to the filter and the source of smoke must be considered during the design process. Where smoke detectors are installed downstream from filters, they should be deemed to serve the purpose of providing an alarm indication of the occurrence of a fire in the HVAC unit (filters, belts, heat exchangers, etc.). These detectors usually serve the purpose of protecting building occupants from the smoke produced by an HVAC unit fire, or smoke ingress via the fresh air intake for the unit. They cannot be expected to serve the purpose of providing detection for the return side of the system.
Where return side detection is required, that requirement should be fulfilled with separate detectors from those monitoring the supply side. In order to be effective, return air duct smoke detectors should be located such that there are no filters between them and the source of the smoke.
Sampling tubes should be oriented to overcome thermal stratification due to buoyancy of the smoke in the upper half of the duct. This condition occurs where duct velocities are low, buoyancy exceeds flow inertia, or the detector is installed close to the fire compartment. A vertical orientation of sampling tubes overcomes the effects of differential buoyancy.
Where a detector is installed on a duct serving a single fire compartment, where the buoyancy exceeds the flow inertia of the air in the duct and the sampling tube cannot be oriented vertically, then the effects of thermal stratification can be minimized by locating the detector sampling tube in the upper half of the duct
The thermal stratification is not a concern where the detector is installed far from the fire compartment or where the smoke is at or close to the average temperature in the duct.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: This portion of the Code was substantially revised for the 2007 edition. For years, the Code had recommended that duct detector sampling tubes be located at least 6 to 10 duct diameters downstream of a bend or change in dimension. Research conducted under the auspices of the Fire Detection Institute discovered that the recommendation had no technical basis.
The research also showed that in most cases detector performance would be improved by mounting sampling tubes in a vertical orientation rather than the horizontal orientation most often seen in actual practice. The vertical orientation would provide for effective sampling when thermal stratification in the duct caused variations in smoke concentration. However, the validity of this generalization becomes less reliable when ducts are encountered that are much wider than they are tall in cross-section.
The requirements in 184.108.40.206.2 and the guidance in A.220.127.116.11.2 are provided to ensure that the detectors in the air duct are suitably located to obtain an adequate sampling of air. These location guidelines should be followed to maximize the probability that smoke will be evenly distributed throughout the duct cross-section at the detector location.
18.104.22.168.3 Detectors shall be mounted in accordance with the manufacturer’s published instructions and shall be accessible for cleaning by providing access doors or control units in accordance with NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: Chapter 14 provides inspection and testing schedules for each type of detector. The accessibility of detectors is critical in order to facilitate cleaning. Poor or neglected maintenance is a dominant cause of unwarranted alarm in smoke detectors.
22.214.171.124.4 The location of all detectors in air duct systems shall be permanently and clearly identified and recorded.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: A permanent placard placed outside the first point of access is advisable to indicate that a detector is accessible from that point. For example, the placard might be mounted on the wall beneath the ceiling tile that must be removed to access the duct. HVAC and fire alarm drawings should clearly show the actual as-built locations of the detectors. In most cases, one drawing that shows only the smoke detector locations is useful. The location can also be included in the display descriptor of addressable systems.
126.96.36.199.5 Detectors mounted outside of a duct that employs sampling tubes for transporting smoke from inside the duct to the detector shall be designed and installed to allow verification of airflow from the duct to the detector.
188.8.131.52.6 Detectors shall be listed for operation over the complete range of air velocities, temperature, and humidity expected at the detector when the air-handling system is operating.
Commentary from the 2010 National Fire Alarm and Signaling Code Handbook: The listing requirements of 184.108.40.206.6 are important to ensure proper operation of a detector in its installed location. Often HVAC system fans and ducts are located in penthouses and mechanical rooms, where comfort heating and cooling are not provided. Consequently, the environment of the detector might exceed the limits observed in the listing investigation. In addition, when warm moist air is circulated through a cold duct smoke detector housing, condensation can occur in the duct smoke detector housing. These conditions can seriously degrade detector performance and stability. Where these extremes are likely, provisions must be made to maintain the operating environment of the detector within its operating range.
220.127.116.11.7 All penetrations of a return air duct in the vicinity of detectors installed on or in an air duct shall be sealed to prevent entrance of outside air and possible dilution or redirection of smoke within the duct.
Subsection 18.104.22.168.8 was deleted by a tentative interim amendment (TIA).