MOUNTING THE AIR DUCT HOUSING
Location on Duct System
Locate the air duct detector in the main supply duct, downstream from the
filters and positioned so as to operate reliably in case of smoke in any part
of the air stream. In instances where filters are capable of removing
smoke, install detectors both upstream and downstream from the filters.
The air duct detectors use sampling tubes which monitor the full width of an
air duct to overcome the limitations of spot-type smoke and heat detectors
in the duct. However, since stratification can occur in the air stream after a
long duct run, locate the detector after bends or inlets which create turbu-
lence, and hence, a more homogeneous mixture of air. The detector should,
when possible, be located a minimum of six duct-widths downstream from the
source of the turbulence (See Figure 1). A 12 inch by 12 inch access hole
should be cut in the duct adjacent to the detector to permit checking and
cleaning of the sampling tubes, if necessary.
4
Figure 1
Typical Mounting of Duct
OUTLET
SAMPLING TUBE
INLET
SAMPLING TUBE
AT LEAST SIX
DUCT WIDTHS
Locate the air duct detector in the air handling system, as shown in Figure
2 and in conformance with NFPA Pamphlet No. 90A, Air Conditioning
and Ventilating System and with NFPA 72 National Fire Alarm Code.
(Both publications are available from the National Fire Protection Associa-
tion, Batterymarch Park, Quincy, Massachusetts.) The detector on the
return air side of the blower should be located at a point prior to exhaust-
ing air from the building or diluting return air with outside air. The detector
on the supply air side of the blower should be downstream of the blower.
Wire or program the detectors into the system so that they automatically shut
down the blowers and operate dampers as required in the event of an alarm.
