Yes. perhaps. Well maybe a little bit wider than you could be.
It depends on the category of the alarm system and to some extent on the fire separation of the ducts. If we are talking about service risers then these could be protected shafts rising through compartments of a building or they could be just holes linking unseparated parts of a building. Then they could connect to horizontal ducts running say in a false ceiling void.
As a rule if you have an L1 system then all protected shafts and voids greater than 800mm will need detction. total coverage.
If you have an L3 system its escape routes and rooms adjacent and opening into escape routes. This could include a service riser that has doors opening onto an escape corridor, but not a service riser that is accessed from a room unless that room itself forms part of the escape route from other areas. You may identify as part of an L2 system specific ducts that pose a significant risk from fire and cover those but not others.
If you have an L4 system then ducts do not need to be protected by detection.
Hope this helps.
