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has the following to say on the use of common return air ductwork for the purpose of exhausting smoke: 3.2.8.8. Mechanical Exhaust System 1) A mechanical exhaust system shall be provided to remove air from an interconnected floor space at a rate of 4 air changes per hour. (See Appendix A.) 2) The mechanical exhaust system required by Sentence (1) shall be actuated by a switch located on the storey containing the entrance for firefighter access referred to in Articles 3.2.5.4. and 3.2.5.5. near the annunciator for the fire alarm system. A-3.2.8.8.(1) Smoke Exhaust System. The mechanical exhaust system is intended as an aid to firefighters in removing smoke and is to be designed to be actuated manually by the responding fire department. Although smoke is normally removed from the top of the interconnected floor space, exhaust outlets at other locations may be satisfactory.
This requires some thought as obviously a fireproofed duct is safer and can work longer. But one needs to think about one's expectations here. If the idea is to have the duct survive being immersed in flames, then the "unexposed side" is both on the inside AND the outside of the duct. That's a tall order! Even chimneys and refractories are allowed to have an unexposed side to cool off. In fire testing, typically, the fire does not touch the item to be tested. The fire stops short of that item. However, the time/temperature curve used is still pretty fierce so it gets very hot. If there's no fireproofing being used on ordinary sheet metal ductwork (like return air), then perhaps what one may have in mind is to exhaust cold or maybe lukewarm smoke, such as at the top of an atrium with a fire way down below. If the duct is sitting in the fire and is expected to exhaust smoke under those circumstances, one needs to think about what the objective is, which should be operability during the fire, meaning that fire may be sucked into that duct, meaning that the duct with its fireproofing is being roasted on two sides. In that case, in particular, there had better be some exposed side criteria for evaluation of tst results. If that duct is Swiss cheese during or at the end of that test because it had trouble with furnace heat inside and out, then there is an open question about operability. If all that is intended is to exhaust lukewarm smoke, then it's no big deal. The intent has to be clear from the start on the part of the mechanical engineer and architect, along with some knowledge of what is tested how. Just because it's listed, does not necessarily mean that it's OK for all purposes wihout qualification. Unfireproofed sheet metal ductwork on its own is not much of a foe to an ASTM E119 fire. Just because the firestop at the point of through penetration holds up, does not necessarily mean that the duct itself is the way the designer intends, if fire has chewed new holes into that duct somewhere.--Achim (talk) 00:22, 24 July 2014 (UTC)[reply]
Sections 909.16.2 and 910.4.4 indicate the same idea as the 2010 NBCC, that the fire department controls this system at the panel.
910.4.3 stipulates a minimum of 2 air changes per hour for the system, as a performance requirement.
Section 910 indicates that there is an overall smoke control system, of which smoke exhaust fans and equipment (ducting could be argued to be part of the equipment) are one aspect of several. Fans have to be rated for 105 degrees Celsius. Control wiring that forms part of a smoke control system must be able to withstand 538 degrees Celsius for a period not less than 15 minutes. Manual controls must override anything automated where the smoke exhaust system is tied into other parts of the ventilation system. That would be return air. Return air equalises pressure, lets (for example) hot air, out of a bedroom ceiling vent, permitting cold air to come in from below. Return air also has an economic feature: energy savings. Not all heat or cold comes in from outside and has to be conditioned. By taking some of the interior air (when there isn't a fire on), where that air has already been heated or cooled for comfort or industrial necessity, is mixed in with freshly conditioned air to cut down on energy costs. As soon as a fire is on, in this scenario, where return air is no longer for energy savings/comfort, it switches from mixing in with supply air to being exclusively for exhaust - to the outside of the building, thus taking smoke out, which helps egress and suppression efforts.
Having said that, building codes provide a minimum life safety regime. Nobody is prevented from going above that to increase life safety, property protection, continuity of operations. Smoke not only threatens life, but also electronics, after a fire, as it can be quite corrosive, which may not be fully realised until days after the fire, when suddenly electronics may require replacement. Seeing to it that any ductwork used exclusively or partly for smoke exhaust survives a fire can be very beneficial.--Achim Hering (talk) 02:54, 28 October 2015 (UTC)[reply]
