Exposed Ceilings vs Plenums

[ME27272727]

Hey everyone. Do open ceiling spaces (ie. rooms with no ceilings, exposed to deck) have the same smoke / flame spread requirements as those in an above ceiling return air plenum? Ultimately, I'm attempting to identify is PVC DWV piping is allowed in a building with open spaces, and whether all other MEP materials need to be plenum rated. This is assuming unitary distributed HVAC in each zone without ducted return, so the entire space is in a way acting as a return plenum.

Thank you.

 

[HVAC-Novice]

A plenum would be coupled to other rooms (plenum above those rooms). If it is just open to structure, i assume you have return ducts. So it isn't a plenum that communicates to other roos or plenums. So that would be treated like anything else in the room (more an architectural question, since they also need to consider smoke/flame spread rating of flooring, wall cover etc.)

 

[nuuvox000]

See chapter 2 of IMC for definition of plenum; it makes it clear.

"An enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement..." etc etc.