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. 83. Access is through the left side blower 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
Smoke Detector Sensor
C08245
Fig. 83 - Typical Supply Air Smoke Detector Sensor
Location
Return Air without Economizer — |
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The sampling tube is located across the return air opening |
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on the unit basepan. See Fig. 86. The holes in the |
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sampling tube face downward, into the return air stream. |
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The sampling tube is connected via tubing to the return air |
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sensor that is mounted on a bracket high on the partition |
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between return filter and controller location. (This sensor |
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is shipped in a |
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requires that this sensor be relocated to its operating |
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location and the tubing to the sampling tube be connected. |
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See “Completing Installation of Return Air Smoke |
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Sensor:” for details.) |
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Return Air with Economizer — |
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The sampling tube is inserted through the side plates of |
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the economizer housing, placing it across the return air | 48TC**16 |
opening on the unit basepan. See Fig. 84. The holes in the | |
sensor that is mounted on a bracket high on the partition | |
sampling tube face downward, into the return air stream. |
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The sampling tube is connected via tubing to the return air |
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between return filter and controller location. (This sensor |
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is shipped in a |
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requires that this sensor be relocated to its operating |
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location and the tubing to the sampling tube be connected. |
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See the following installation procedure.) |
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Flexible
Exhaust Tube
Return Air
Sampling Tube
C10330
Fig. 84 - Return Air Sampling Tube Location
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