Can a duct-type smoke detector be installed outdoors?
NFPA 72 allows smoke detectors to be installed outdoors as long as the detector is listed and designed for the expected conditions. When installed outdoors, the detector should be rated within the temperature ranges expected at the location, and, shall be installed in a weather proof enclosure. Further more, smoke detectors should operate reliably under such common environmental conditions as mechanical vibration, electrical interference, and other environmental influences.
Below is picture of a duct detector installed on a roof (open to weather) of a school to comply with the Building Code fan shut-down requirements. To allow the smoke detector to remain in the rated temperature range, the installer provided a heating pad (pictured below). The heating pad is automatically switched by a 24V thermostat located adjacent to the duct detector. Unfortunately, the heating pad should have not been installed on the base (lowest point) of the enclosure as the enclosure could accumulate humidity and short out the wiring of the coil.
NFPA 72 17.7.1.8* Unless specifically designed and listed for the expected conditions, smoke detectors shall not be installed if any of the following ambient conditions exist
(1) Temperature below 32°F (O°C)
(2) Temperature above 100°F (38°C)
(3) Relative humidity above 93 percent
(4) Air velocity greater than 300 ft(min (1.5 mlsec)
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