27
Smoke Detector Sensor
The Smoke Detector Sensor (see Fig. 35) includes a
plastic housing, a printed circuit board, a clear plastic
cover, a sampl ing tube inlet and an exhaust tube. The
sampling tube (when used) and exhaust tube are attached
during installation. The sampling tube varies in length
dependi ng on the size of the rooftop unit . The clear plastic
cover permits visual inspections without having to
disassemble the sensor. The cover attaches to the sensor
housing using four captive screws and forms an airtight
cham ber around the sensing electronics. Each sensor
includes a harness with an RJ45 terminal for connecting to
the controll er. Each sensor has four LEDs (for Power,
Trouble, Alarm and Dirty) and a manual test/reset butt on
(on the left--side of the housing).
Air is introduced to the duct smoke detector sensor’s
sensing chamber through a sampling t ube 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 sampled air through the sensing chamber. When a
sufficient amount of smoke is detect ed 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 environmenta l 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 sampled air through the sensing chamber. When a
sufficient amount of smoke is detect ed 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
dete ctor doe s not differentiate which sensor signals an
alarm or trouble c ondition.
SEE DETAIL A
DETAIL
A
PLUG
SAMPLING TUBE
(ORDERED 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)
C08209
Fig. 35 -- Smoke Detector Sensor
Smoke Detector Locations
Supply Air: The Supply Air Smoke Detector Sensor is
located to the left of the unit’s indoor (supply) fan. See
Fig. 35. Access is through the fan access panel. There is
no sampling tube used at this location. The sampling tube
inlet extends through the side plate of the fan housing
(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.
SUPPLY AIR
SMOKE DETECTOR
C08245
Fig. 36 -- Typical Supply Air Smoke Detector Sensor
Location
48HC
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