4
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 turbulence, 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 de-
tector to permit checking and cleaning of the sampling tubes, if necessary.
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
Figure 2
Recommended Locations in Duct Systems
Figure 1
Typical Mounting of Duct
OUTLET
SAMPLING TUBE
INLET
SAMPLING TUBE
AT LEAST SIX
DUCT WIDTHS
