Above ceiling as return plenum 1

[gatt]

Is it normal to use the area above a suspended ceiling as a return plenum? We have a large building with several rooms and zones with no return ducting at all. It seems to me we are picking up a lot of heat in the summer (losing heat in winter). Just curious.

 

[VSSriram]

It's quite normal to use the ceiling void as a return air plenum. It saves on return air ducting as well as the thickness of insulation required for the supply air ducting. If it's ducted return, then the ceiling void can get quite warm/hot and if the insulation is not done properly, (more so, if there is exposed roof above) can cause condensation problems.

 

[lilliput1]

It also reduces the supply air requirement because the plenum heat gain goes to the return airstream, not to the conditioned space. However note the following:
-Wiring in the plenum must be plenum rated (Teflon insulated)
-Partition must not go all the way to the slab above unless transfer air openings are provided
-Transfer air openings must be sized for minimal pressure drop, about maximum 300 FPM velocity to minimize pressurization to effect transfer air flows. Sometimes a transfer fan may be required.
- Maintain fire/smoke rating of rated walls by providing fire or combination fire & smoke dampers where required
- Avoid sound transmission. May have to use transfer air silencers or ducts with elbows & acoustic lining.

 

[ChasBean1]

I agree with both above. It's very normal. At the same time partitions and above-ceiling wall separations and requirements for fire rating systems become more important.