air plenum or return duct

[317069]

Hello all

If we have a floor includes different units with different tenants,it is served by one AHU.
Which one is better?
- install a suspending ceiling in each unit to create return air plenum then connect all units together by making openings in unit’s walls with fire dampers
OR
- using a return duct, also with fire damper when it penetrate the walls between units
Thank you
Attached is a sketch

 

[willard3]

Plenums are next to impossible to balance when they are new and, after a couple of years and occupant changes, nothing will balance.

Ducted returns can be balanced.

 

[sprinkler1000]

Hello:

If installing one fancoil per unit is not an option, I would prefer having a return duct going through a common corridor and getting into each room. It is better from an acoustic point of view and better for balancing.
Fire dampers are necessary depending on the fire zone criteria of the building.

 

[DRWeig]

Small project like this, especially with leased-out space, I'd also vote to duct the return. Tenants don't often consult with an engineer before stringing a bunch of non-plenum-rated cabling above a ceiling for their home-grown computer and security networks.

Them's my two cents' worth.

Goober Dave