The current NBC 95 as well as the OBC-97 incorporated a very subtle change to the clause 3.2.4.12 "Prevention of Smoke Circulation". The word "recirculating" was changed to now read "circulating". The added criteria of serving more than one storey, more than one suite in a storey or more than one fire compartment still applies.
By making this very subtle change, we are now required to provide duct detectors on all supply air systems as well as those recirculating systems which previously required detectors. Operation of the duct detector requires that the fane system be arranged for automatic shut down. In some buildings, including residential where the corridors are normally provided supply air, this added requirement could be significant.
Implementing this change to the Code makes a great deal of sense. In 1991 ULC S 524 was modified to provide duct detectors only on the supply side of fan systems, mounted downstream of the makeup air connection, the fan and the filters. This was in recognition of the stated intent of ensuring that smoke was not spread to other floors or areas in the building via the air handling system regardless of its source.
Common sense therefore suggests that contamination of any of the items noted above, will affect a supply air system in the same manner as a recirculating system, which justifies this change
Generally, if a supply air system is not used for smoke control, we believe it is a good idea to leave the fan running when there is a system alarm and shut down the fan system only on the operation of the duct detector serving that system. This will stop the duct which will have openings on many floors from becoming a shaft and spreading smoke to other floors. Manual control for shutdown could be included on the annunciator to allow the fan to be shut down manually in the event that investigating personnel or the firefighters consider it necessary.