Fire Detection Institute | Fire Protection Engineering
Does duct smoke detection really work?
Is it worth the cost? Is the potential for false and nuisance alarms
worth the added protection? The universal answer to all fire protection
questions is: It depends. This is the first part of a two-part article
that summarizes the findings of research recently conducted under the
auspices of the Fire Detection Institute. This article summarizes the
purpose of the research and findings in the areas of smoke-driving
forces, smoke dilution in ductwork, and the effects of smoke aging on
detection in ductwork. Part 2, which will appear in the Spring 2006
issue, will summarize findings regarding the effects of HVAC filters on
smoke detection in ducts, smoke stratification in ducts, and the
efficacy of duct detectors that use sampling tubes.
Building, fire, and life safety codes
often require the installation of duct smoke detectors in heating,
ventilating, and air conditioning (HVAC) ducts for the detection of
fires in the HVAC system. Typically, these codes refer to NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems,1to determine if and where they are required. NFPA 90A in turn refers to the National Fire Alarm Code2
for specific design, installation, and maintenance criteria for these
detectors. There is no known research or data analysis that supports the
prescriptive criteria in the National Fire Codes. Historically, these
criteria reflect the engineering judgment of experienced practitioners
in the field.
In an effort to support the committees and system designers,
the Fire Detection Institute undertook a research program to study duct
smoke detection and to provide technical data to the committees and
system designers. The Fire Detection Institute Duct Detection Research
Initiative (FDI/DDRI) employed the research assets of the Department of
Fire Protection Engineering of the University of Maryland (UMDFPE) and
the National Research Council Canada (NRC). The goals were to either
support, change, or eliminate duct smoke detection requirements from the
codes and standards, and to study the many myths about duct smoke
detection. That research program has been completed, and reports have
been issued. This article is a short summary of that work.
The issues identified for research
related to both the use of duct smoke detectors for detection of fires
within the HVAC system and as part of a larger smoke management system.
Six issues were identified for investigation:
Comparative Driving Forces (fire vs. fans)
Effects of Smoke Aging and Condensation
within the HVAC System on the Performance of Duct Smoke Detectors
Effects of HVAC Filters on the Performance of Duct Smoke Detectors
Stratified Flow in HVAC Ducts (effect on detector or sampling tube location)
Efficacy of Duct Detectors using Sampling Tubes
There are two principle uses of duct
smoke detection that had to be considered in the research plan. The
first was detection of smoke resulting from a fire in the building
served by the HVAC system. That is, a fire external to the HVAC system.
The second principle use of duct smoke detection is detection of smoke
originating in the HVAC system, such as a filter fire, or smoke
originating outside the building in close proximity to a fresh-air
The research included surveys of existing buildings and HVAC systems;
fire, smoke, and air movement modeling; and full-scale fire testing.
Modeling done by the UMDFPE team utilized a hypothetical 10-story
building. The analytical methods they used and reported on are valid for
smaller and larger buildings. The NRC team ran full-scale tests in a
real 10-story building.
Comparative Driving Forces
The test hypothesis was that it was
unnecessary to shut down the HVAC system because the driving forces due
to the fire were large compared to those of the mechanical ventilation.
There was a concern that the pressurization produced by the fire
constituted a driving force on the air in the HVAC system ductwork that
was large compared to that produced by the HVAC system fan(s). If this
were the case, the fire would overpower the HVAC system, rendering the
interactive control of the HVAC fans superfluous. This suggested that
considerable expense was being incurred for the installation and
maintenance of duct smoke detectors that did not provide a commensurate
In addition to the movement of smoke caused by buoyancy, the
research also addressed air movement due to stack effect and wind.
The results indicate that the pressures
produced by the HVAC system were generally larger than those produced by
the fire. Since the HVAC system produced higher pressures, it also
produced higher flows and was more effective in distributing smoke to
the nonfire floors than the passive buoyancy, stack, and wind effects.
Consequently, permitting the HVAC system
to continue to run normally during a fire will generally result in
higher smoke concentrations on nonfire floors and in nonfire
compartments than if the mechanical ventilation system is shut down,
allowing smoke to flow subject only to the pressures resulting from the
passive buoyancy, stack, and wind effects. This suggests that, unless an
active smoke management strategy is in place, the HVAC systems should
be shut down upon detection of a fire. The extent of this advantage
depends upon the specific characteristics of the building in question,
and it can be quantified by using the computational methods outlined in
the UMDFPE report.
The analysis of the comparative driving forces
demonstrated that the requirements for duct smoke detectors in NFPA 90A do
have a technical basis. Consequently, these requirements should remain
as part of the minimum compliance prescriptive standard. Where the
design fire is substantially greater than the nominal 100 kW to 500 kW
fire, or where the construction is substantially atypical, a
performance-based analysis can be performed using the UMDFPE report as
an outline guide.
One concern is whether the smoke
concentration in the HVAC ducts would be too low to produce reliable
detection in response to the fires they are generally expected to
detect. The second concern was whether the dilution of the smoke by
fresh air in the ductwork is so severe that duct smoke detection is only
effective in responding to fires that would be characterized as a
The UMDFPE team conducted laboratory tests
using a medium-scale cone calorimeter and duct system, and developed
analytical relationships. The NRC team conducted full-scale tests in the
10-story test building. The data from the NRC experiments support the
relations developed by UMDFPE. As expected, the dilution of the smoke
was proportional to the ratio of the air flows from the fire room
(floor) and the nonfire rooms (floors). The NRC tests also demonstrated
that, even with substantial dilution ratios, the commercially available
duct smoke detectors were able to detect fires on the order of 100 kW to
200 kW. Thus, while dilution is important, it does not obviate the
ability of commercially available duct-type smoke detectors to perform
Smoke Aging Effects
As smoke moves away from a fire source,
it cools and changes characteristics. Condensation, agglomeration, and
coagulation result in changes to smoke particle size, shape, and color.
Soot deposition on surrounding surfaces reduces the amount of smoke in
the air stream. These changes affect the response of commercial smoke
The UMDFPE team studied smoke aging using the duct on the
mediumscale cone calorimeter and a laser sheet apparatus to measure
smoke particle size and number. That work showed that the number of
small particles at the duct inlet, compared to 3 m downstream, decreased
by a factor of 10 while the number of large particles increased by a
factor of two. This suggests that the aging of smoke occurs more rapidly
than might have been expected. For all but two test scenarios,the NRC
tests in the full-scale building showed very little difference in the
response of analog smoke detectors that were in the ductwork and
separated by 10 stories. The tests that varied allowed the smoke to
recirculate through the ductwork for an extended period. After about 12
minutes, there was some change in detector response. Those tests would
not be representative of the real use of duct smoke detection.
Thus, it appears that the aging of smoke
in ductwork occurs predominantly in the first few meters. Therefore,
duct smoke detectors that are located some distance from the source of
smoke are not adversely affected by smoke aging. If the duct smoke
detectors are of the photoelectric type, the response may be improved
since the detector response is proportional to the particle diameter
raised to the 2nd to 6th power, but only linearly proportional to the
number of particles.4 However, this expected increase in
detector signal may be offset by soot deposition on the duct walls. This
may explain the lack of difference in detector signal in the NRC
NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, National Fire Protection Association, Quincy, MA, 2002.
NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2002.
Mnizewski, K., and Waterman, T.E., "The
Effectiveness of Duct-Installed Smoke Detectors in Two Different
Ventilation System Configurations," IITRI Report for the GAS, February
Schifiliti, R.P., and Pucci, W.E., Fire Detection Modeling: State of the Art, the Fire Detection Institute, Bloomfield, CT, 1996.
Editor's Note About This Article
This is a continuing series of articles
that is supported by the National Electrical Manufacturer's Association
(NEMA), Signaling Protection and Communications Section, and is intended
to provide fire alarm industry-related information to members of the
fire protection engineering profession.
The Fire Detection Institute is a
nonprofit corporation dedicated to improving the fire detection and
alarm standards through testing programs and research. The institute has
sponsored research that led to the development of heat and smoke
detection modeling and research on the effects of ceiling shape and
geometry, and the effects of HVAC on the response of room detectors. The
Duct Smoke Detection research project was sponsored by contributions
and grants from foundations, manufacturers, and interested parties.
Copies of a more complete summary are available through the Automatic
Fire Alarm Association at www.afaa.org.
The full reports for both teams and a full set of all data collected
are available from the Society of Fire Protection Engineers through
their online store at www.sfpe.org. For information about the Fire Detection Institute, contact the FDI Secretary at firstname.lastname@example.org.