22
Smoke Detector Sensor
The smoke detector sensor (see Fig. 39) includes a plastic
housing, a printed circuit board, a clear plastic cover, a sam
-
pling tube inlet and an exhaust tube. The sampling tube (when
used) and exhaust tube are attached during installation. The
sampling tube varies in length depending 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 air
-
tight chamber around the sensing electronics. Each sensor in
-
cludes a harness with an RJ45 terminal for connecting to the
controller. Each sensor has four LEDs (for Power, Trouble,
Alarm and Dirty) and a manual test/reset button (on the left-
side of the housing).
Fig. 39 — Smoke Detector Sensor
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
sampled 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
sampled 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.
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. 40. 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. 40 — 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. 41. The holes in the sampling tube
face downward, into the return air stream. The sampling tube is
connected through tubing to the return air sensor that is mount
-
ed on a bracket high on the partition between return filter and
controller location. The sensor is shipped in a flat-mounting lo
-
cation. Installation requires that this sensor be relocated to its
operating location and the tubing to the sampling tube be
connected. See installation steps.
Fig. 41 — Typical Return Air Smoke Detector Location
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)
SUPPLY AIR
SMOKE DETECTOR
RETURN AIR
SMOKE DETECTOR
(AS SHIPPED)
Summary of Contents for Preferred 549K 04-06
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