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
101:9.6.3.2.2* 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 1:13.7.1.4.10.2.3*]
A.9.6.3.2.2 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.13.7.1.4.10.2.3]
FFPC
101:9.6.5 Fire Safety Functions.
9.6.5.1 Fire safety
functions shall be installed in accordance with the requirements of NFPA 72,
National Fire Alarm and Signaling Code. [FFPC 1:13.7.1.4.12.1]
9.6.5.2 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 9.6.5.2(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.
FLORIDA
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
systems.
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.
3.
An
area smoke detector system as prescribed in the exception to Section
606.2.1.
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.
606.3
Installation.
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.
606.4.1 Supervision.
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.
Exceptions:
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.
6.4.2* Location.
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).
6.4.2.1 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
6.4.2.2 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.
6.4.2.3 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.
6.4.3* Function.
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.
6.4.3.1 Smoke detectors
provided as required by 6.4.2 shall automatically stop their respective fan(s)
on detecting the presence of smoke.
6.4.3.2 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.
6.4.4 Installation.
6.4.4.1 Smoke detectors shall
be installed, tested, and maintained in accordance with NFPA 72, National Fire Alarm and Signaling Code.
6.4.4.2 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.
6.4.4.2.1 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.
6.4.4.3 Where smoke detectors
required by Section 6.4 are installed in a building not equipped with an
approved fire alarm system as specified by 6.4.4.2, 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.
6.4.4.4 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).
17.7.4.1* In spaces
served by air-handling systems, detectors shall not be located where airflow
prevents operation of the detectors.
A.17.7.4.1 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
17.7.4.1 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.17.7.4.1 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).
17.7.4.2 Detectors
installed in plenums shall comply with 17.7.4.2.1 and 17.7.4.2.2.
17.7.4.2.1 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
17.7.1.8. 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
17.7.1.8 and A.17.7.1.8.
17.7.4.2.2* 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.17.7.4.2.2 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.
17.7.5.1*
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.17.7.5.1 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 17.5.3.1
identifies all the spaces that must have smoke detectors if total coverage is
to be achieved.
17.7.5.2*
Limitations.
A.17.7.5.2 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.
17.7.5.2.1 Detectors
that are installed in the air duct system in accordance with 17.7.5.1(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 17.7.5.2.1 specifically prohibits
the use of duct smoke detection in lieu of area detection installed pursuant to
Section 17.7.
17.7.5.2.2 Where open
area protection is required, 17.7.3 shall apply.
17.7.5.3*
Purposes.
A.17.7.5.3 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
17.7.5.3.1 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 17.7.5.4.2.1.
17.7.5.3.2 If smoke
detectors are used to initiate selectively the operation of equipment to
control smoke spread, the requirements of 17.7.5.4.2.2 shall apply.
17.7.5.3.3 If detectors
are used to initiate the operation of smoke doors, the requirements of 17.7.5.6
shall apply.
17.7.5.3.4 If duct
detectors are used to initiate the operation of smoke dampers within ducts, the
requirements of 17.7.5.5 shall apply.
17.7.5.4
Application.
17.7.5.4.1
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 17.7.5.4.1 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 17.7.5.4.2.2(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
17.7.5.4.1 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.
17.7.5.4.2*
Smoke Detection for the Air Duct System.
A.17.7.5.4.2 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.
17.7.5.4.2.1
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 17.7.5.4.2.1 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 17.7.5.4.2.1 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]
6.4.2.1 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:6.4.2.1]
6.4.2.2 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:6.4.2.2]
6.4.2.3 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:6.4.2.3]
17.7.5.4.2.2*
Return Air System. Unless otherwise modified by 17.7.5.4.2.2(A)
or 17.7.5.4.2.2(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 17.7.5.4.2.2(A) is
based on the same reasoning used in the exception to 17.7.5.4.2.1. With
reference to Figure A.17.7.5.4.2.2(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
17.7.5.4.2.2(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.17.7.5.4.2.2 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.17.7.5.4.2.2(a) through Figure
A.17.7.5.4.2.2(c)]:
(1)
Width.
(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.17.7.5.4.2.2(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.17.7.5.4.2.2 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.
17.7.5.5
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 17.7.5.5, 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.
17.7.5.5.1 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.
17.7.5.5.2* 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 17.7.5.5.2(1) and 17.7.5.5.2(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.17.7.5.5.2 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 17.7.5.4.2.2 take precedence over these
considerations. [See Figure A.17.7.5.5.2(a) and Figure A.17.7.5.5.2(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 17.7.5.5.2 and the guidance in A.17.7.5.5.2 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.
17.7.5.5.3 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.
17.7.5.5.4 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.
17.7.5.5.5 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.
17.7.5.5.6 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 17.7.5.5.6 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.
17.7.5.5.7 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
17.7.5.5.8
was deleted by a tentative interim amendment (TIA).