This is a reasonable conclusion if the smoke detector is exposed to similar conditions as the UL tests used for approval or the series of tests conducted by NIST for specific residential fire scenarios. However, significant differences in the fire scenario or location of the detector relative to the fire or building components do not support such a conclusion. In this case, the SD is on the underside of a timber ceiling with the fire in the space above. This is well outside the conditions of the previously referenced tests. In fact, smoke detectors are not currently designed and listed for this scenario.

If one assumes that the fire starts in the space above the timber ceiling and there is relatively little ventilation compared to the required ventilation to support the growing HRR, then the fire would quickly consume the available oxygen within the volume of the space. The fire would then be regulated to the level of HRR that could be supported by any ventilation flow through vents and leakage paths. Let's also assume that the smoke in the space is of sufficient temperature to char the timber ceiling. If these are individual timber slats that make up the ceiling, they have corners that provide a better surface to mass ratio that will provide a faster rate of charring than the flat surface of the wood. So the charring would first produce open slots between each individual slat that can grow during the fire. I am sure you all have observed this type of damage to these types of structures at fire scenes.

Assuming that vents are installed in the space above for ventilation, the predominate flow through the charred slots would generally be upwards. Thus, the smoke detector is not being exposed to smoke in the same manner as they are designed for. At the same time, the slots may provide some radiant heat transfer to take place from the hotter space above to the smoke detector on the underside of the ceiling. This may cause significant heating of the detector to occur prior to any exposure to significant smoke that may be sufficient to activate the smoke detector.

If the space above the timber ceiling can remain at a higher pressure due to a lack of adequate vent area, then you may get some smoke to flow due to pressure differences into the space under the timber ceiling under the right conditions. Remember that the activation of the smoke detector is dependent on both the smoke concentration as well as the local velocity at the detector. There is an entry resistance associated with smoke detectors that can provide a delay in activation even though the smoke concentration outside the detector is sufficient to cuase activation. So in the scenario provided, the smoke concentration and velocity at the detector is highly dependant on the ventilation flows that are sensitve to the changing flow conditons during the fire. Thus, the bottom line is that "cda" is correct that "it depends", although the proceeding discussion does not conservatively rule out that a smoke detector in this scenario could be thermally damage prior to it activating from exposure to smoke.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com