31
Air is introduced to the duct smoke detector sensor’s sensing
chamber through a sampling tube that extends into the HVAC duct
and is directed back into the ventilation system through a (shorter)
exhaust tube.
The difference in air pressure between the two tubes pulls the sam-
pled air through the sensing chamber. When a sufficient amount of
smoke is detected in the sensing chamber, the sensor signals an
alarm state and the controller automatically takes the appropriate
action to shut down fans and blowers, change over air handling
systems, notify the fire alarm control panel, etc.
The sensor uses a process called differential sensing to prevent
gradual environmental changes from triggering false alarms. A
rapid change in environmental conditions, such as smoke from a
fire, causes the sensor to signal an alarm state but dust and debris
accumulated over time does not.
The difference in air pressure between the two tubes pulls the sam-
pled air through the sensing chamber. When a sufficient amount of
smoke is detected in the sensing chamber, the sensor signals an
alarm state and the controller automatically takes the appropriate
action to shut down fans and blowers, change over air handling
systems, notify the fire alarm control panel, etc.
For installations using two sensors, the duct smoke detector
does not differentiate which sensor signals an alarm or trouble
condition.
Fig. 40 — Smoke Detector Sensor
Smoke Detector Locations
SUPPLY AIR
The supply air smoke detector sensor is located to the right of the
unit’s indoor (supply) fan. See Fig. 41. Access is through the fan
access panel. The sampling tube inlet extends through the fan
deck (into a high pressure area). The controller is located on a
bracket to the right of the return filter, accessed through the lift-
off filter panel.
Fig. 41 — Typical Supply Air Smoke Detector Sensor
Location
RETURN AIR SMOKE DETECTOR SENSOR WITHOUT
ECONOMIZER
The sampling tube is located across the return air opening on the
unit basepan. See Fig. 42-43. The holes in the sampling tube face
downward, into the return air stream. The sampling tube is con-
nected through tubing to the return air sensor that is mounted on
a bracket high on the partition between return filter and control-
ler location. The sensor is shipped in a flat-mounting location.
Installation requires that this sensor be relocated to its operating
location and the tubing to the sampling tube be connected. See
installation steps.
Fig. 42 — Typical Return Air Smoke Detector
Location (size 8-14 units)
See
Detail A
Detail A
Plug
Sampling Tube
(Covered Separately)
Coupling
Magnetic
Test/Reset
Switch
Alarm
Trouble
Power
Dirty
Sensor
Cover
Cover Gasket
(Ordering Option)
Sensor Housing and Electronics
Exhaust Gasket
Exhaust Tube
Duct Smoke Sensor
Intake
Gasket
TSD-CO2
(Ordering Option)
Return Air
Smoke Detector
(As Shipped)
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