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Core Theme
This content explains how to properly determine the spacing for heat detectors in fire alarm systems, emphasizing the critical role of NFPA 72 standards and environmental factors in ensuring effective coverage and code compliance.
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Listed spacing for a heat detector is
the maximum spacing established during
testing, showing how far apart detectors
can be placed on a smooth low ceiling.
There are two methods for applying
listed spacing. The square layout and
the circular layout. In square layout
method, detectors are placed in a square
grid pattern. The distance between
detectors is equal to the listed
spacing. Each detector covers a square
area. For example, with 50 ft spacing,
50 ft * 50 ft equals 2500 ft. In the
circular method, every point on the
ceiling shall be within 0.7 times listed
spacing from a detector. For example, a
50 ft listed spacing detector radius is 0.7*
0.7*
50 equals 50 ft. Depending on the
architecture, square layout suits
grid-like spaces and may require fewer
detectors, while circular layout is
ideal for irregular areas and corridors,
often maximizing coverage with fewer
devices. As you can see in this
irregular space example, the circular
area method reduces the number of
detectors. With the circular method,
only five detectors are needed, while
the square method requires seven
detectors to properly cover the area.
Heat detectors are typically spaced
according to standards established
through full-scale fire tests conducted
by underwriters laboratories. These
tests determine the listed spacing, the
maximum allowable distance between
detectors under ideal conditions. The UL
test installs standard automatic
sprinkler heads 10 ft by 10 ft spacing
around the alcohol pan. The heat
detectors are placed around the
automatic sprinkler heads fuses becomes
the listed spacing of the heat detector.
Real world environments often presents
challenges that affect heat detection
performance. High ceilings, structural
elements like beams and joist can
interfere with heat movement and delay
detector activation. To address these
variables, NFPA72
requires that actual installed spacing
be less than the listed spacing when
such conditions are present. Listed
spacing is established by the
manufacturer and reflects the detector's
ability to reliably respond to thermal
conditions across a defined coverage
area. UL521
evaluates the detector's sensitivity,
response time, and operational
reliability under controlled fire
scenarios. The adjusted spacing is
referred to as the design spacing or SD.
If no environmental adjustments are
necessary, design spacing equals listed
spacing. But when conditions vary, NFPA72
NFPA72
provides specific formulas and reference
tables to calculate the appropriate
reduced spacing. [Music]
[Music]
When designing heat detector layout in
accordance with NFPA72,
determining the correct spacing is
critical to ensure code compliance. This
process involves three steps. It starts
with the listed spacing. Step two is to
apply the ceiling height reduction
factor. As ceiling height increases,
more ambient air is entrained into the
plume, making it cooler. The higher the
ceiling, the smaller the allowable
distance between detectors. Step three
is to apply surface adjustment factor.
Architectural features like beams and
joists can obstruct heat flow. After
applying both the ceiling height and
surface adjustment factors, we arrive at
the final value known as design spacing
SD design spacing is always less than or
equal to the listed spacing ensuring
that system is working properly on the
real world conditions. [Music]
[Music]
When designing heat detection systems,
ceiling height plays a critical role in
determining detector spacing. As a fire
plume rises, it entrains surrounding
ambient air. This process cools the
gases and dilutes the concentration of
fire products, making it harder for heat
detectors to respond quickly. To
calculate this adjustment, we refer to
table 176351
in NFPA72.
This table provides reduction factors
that must be applied to the
manufacturer's listed spacing. For
example, let's say a heat detector has a
listed spacing of 50 ft at a ceiling
height of 20 ft. Table 176351
specifies a reduction factor of 0.64.
So, the adjusted spacing becomes SD=
0.64 * 50, which is 32 ft. In addition
to spacing reductions for high ceilings
and complex surfaces, code also defines
a lower boundary for how closely heat
detectors should be placed. The minimum
spacing of heat detectors shall not be
required to be less than 0.4 times the
height of the ceiling. [Music]
[Music]
The type of ceiling construction plays a
critical role in how combustion products
move and how effectively heat detectors
respond. NFPA72
classifies ceilings into three main
categories: smooth ceiling, beam
construction, and solid joist construction.
construction. [Music]
[Music]
Joists slow down the ceiling jet, the
horizontal movement of hot gases,
especially in the direction
perpendicular to the joists. To
compensate for this, the code mandates a
reduction in spacing. The listed spacing
perpendicular to the joists must be
reduced by 50%.
This ensures detectors are close enough
to respond effectively despite the
slowed heat movement. No spacing
reduction parallel to the joists is
required. Since heat can flow more
freely along the joist channels as per NFPA72
NFPA72
section 176 322 under joisted ceilings,
heat detectors should be mounted at the
bottom of the joists. [Music]
[Music]
When beams project over 4 in and are
spaced more than 3 ft apart, they block
the ceiling jet. Code requires detector
spacing perpendicular to the beams be
reduced to 2/3 of the listed spacing
while spacing parallel to beams remains
unchanged. When ceiling beams are very
deep and spaced far apart, heat movement
is significantly obstructed. According
to NFPA72
section 1763313,
if beams are greater than 18 in and
spaced more than 8 ft apart, each beam
pocket shall have its own detector. This
ensures no pocketed area is left
unprotected as heat may collect within
each bay without spreading quickly to
heat detectors can be mounted on the
beam bottoms when beams are under 12 in
deep and spaced less than 8 ft apart
center to center. [Music]
[Music]
The heat detector app in the NSVC CAD
fire alarm module helps users design
code compliant detector layouts by
calculating design spacing. It
automatically adjusts for ceiling height
and structural obstructions like beams
and joists to ensure accurate placement.
To begin, users should define the
building structure and walls using the
define structure app. In this
demonstration, we will define the
joists, assuming the other walls and
construction elements are already specified.
specified.
To run the app, click the icon or type
NHD. The app interface will then appear,
prompting you to select the unit of
measurement for your layout, the
applicable NFPA
72 edition, the system type, and the
detector orientation. Next, input the
listed spacing of the detector and
ceiling height. Once you've entered all
the necessary data, click the insert
detector button to place the detector on
your drawing. When the detector is
positioned in an area with joist
construction, the software automatically
adjusts the spacing by 50% perpendicular
to the joist. Additionally, the app
ensures that the detector is placed
directly beneath the joist to meet the
requirements of section 17632
of the NFPA72 code.
In areas with beams, the software
automatically reduces the spacing of
spot type heat detectors to 2/3 of the
listed spacing perpendicular to the beam direction.
for beams exceeding 18 in in height and
spaced over 8 ft apart. The app limits
the detector's coverage area to each bay.
bay.
In corridors with smooth ceilings and
regular geometry, the software
automatically adjusts the detector's
coverage area to an optimal detection
radius. Users can modify the coverage
shape by pressing S for square or C for
circle on their keyboard. Pressing the
control key opens a pop-up window that
displays relevant information, including
the required number of detectors based
on either the circular or square layout
The software intelligently analyzes room
geometry and layout to optimize
detectors fully aligned with the
requirements of NFPA72
and project specifications, ensuring
reliable coverage, avoiding overlap and
reducing installation costs.
I hope you find this video helpful. You
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