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4. DETECTOR INSTALLATION
After the most suitable detector for the job
is selected, the location of the detector
within the space to be protected should be
considered.
Spot type detectors are usually located on
the ceiling or side wall with the edge of the
detector located no closer than 4 inches
from the wall or ceiling. When heat detec-
tors are installed at their listed spacing,
detection times are approximately equiva-
lent to the operating time of standard 165° F
link-and-lever sprinklers. If faster response
is desired, reduce detector spacing. Also,
where ceiling heights exceed 16 feet, or
where ceiling construction is not smooth,
reduce spacings accordingly. Specific
information on the treatment of joisted,
beamed, and sloped ceilings can be found
in Annex A of NFPA 72 (2002 Edition).
When installing any type of heat detector,
consideration should be given to sources
of heat within the protected space which
might cause false alarms. For example,
locate heat detectors away from unit
heaters and ovens where surges of hot air
might be expected.
The installation of smoke detectors is more
critical than for heat detectors because
smoke transport is strongly influenced by
the convective air flow patterns within the
Section V — Detector Application and Spacing
protected area. For this reason, smoke
detectors are not assigned a listed spacing
by the testing laboratories other than a
maximum area coverage of 900 square feet
per detector. Although a grid pattern can be
used as a starting point, care must be taken
to appropriately locate the heating supply
registers and return air registers. Smoke
detectors should be located away from
turbulence caused by hot air outlets. The
location of the smoke detectors should favor
return air because the return air draws
smoke toward the detector and because air
velocity at the return tends to be lower.
Smoke stratification also should be consid-
ered when smoke detectors are installed.
Smoke may stratify below a ceiling due to
temperature gradients or air flow along the
ceiling. When stratification is a possibility,
smoke detectors can be installed with alter-
nate detectors at different levels.
The installation of ionization type detectors is
similar to that of smoke detectors since fire
gases tend to flow with smoke and are
similarly affected by convected flows within
the protected space. They also must be
located away from sources of oxidizable
gases or vapors; for example, away from
substances like aerosol sprays or hydrocar-
bon solvents, as these substances could
cause false alarms.
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