Carrier 50TCQD17-D24 Service And Maintenance Instructions page 27

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.
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 left of the unit's indoor (supply) fan. See
Fig. 41. 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
SENSOR
Fig. 41 - - Typical Supply Air Smoke Detector Sensor
Location
Return Air without Economizer — The sampling tube is
located across the return air opening on the unit base pan.
See Fig. 42. The holes in the sampling tube face
downward, into the return air stream. The sampling tube is
connected by tubing to the return air sensor that is
mounted on a bracket high on the partition between return
filter and controller location. (This 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 "Completing
Installation of Return Air Smoke Sensor" for installation
steps.)
Fig. 42 - - Typical Return Air Detector Location
Return Air with Economizer — The sampling tube is
inserted through the side plates of the economizer
housing, placing it across the return air opening on the
unit base pan. See Fig. 43. The holes in the sampling tube
face downward, into the return air stream. The sampling
tube is connected via tubing to the return air sensor that is
mounted on a bracket high on the partition between return
filter and controller location. (This 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 "Completing
Installation of Return Air Smoke Sensor" for installation
steps.)
C10325
Fig. 43 - - Return Air Sampling Tube Location
27
RETURN AIR DETECTOR
SAMPLING TUBE
C09135
RETURN AIR DETECTOR
SAMPLING TUBE
C09136