The air conditioning system of an apartment, in a 8 story building - each floor is consist of one apartment 400 m2, consists of a separate split unit for each room in the apartment. the dining and living room, which have no partition between them, are served with a separate package unit. All AC outdoor units of each apartment are installed in the balconies on the same level and the indoor units are installed above the ceiling of each room's toilet. The supply air of each unit is ducted, whereas the return air is open i.e the space above the ceiling is plenum. The question is Can I use nonmetallic conduits above ceiling (The ceiling is fire rated)? I think it should be OK, since there is no duct or plenum connecting the different spaces in the apartment. Each space is served with a separate unit and the walls between the rooms are up to the structural slab and hence there is no chance to convey smoke or fire between the rooms of the apartment.