Return air ceiling plenum?

[remp]

I have a large ceiling void that I want to use for return air plenum instead of ducting directly from the room. The problem is the plenum top section is a roof made from metal decking, insulation, plywood, rubber. Its pretty old but looks in perfect condition.
Is it ok to use this space as a plenum given that the roof may not be 100% sealed?

 

[Wareagl487]

Assuming all material in the plenum space is of non-combustible construction (including wiring), I would have a problem doing it.

Andy W.

 

[AbbyNormal]

climate can be a factor in addition to the issue of combustibles, but if you are anywhere humid you will be creating a large negative pressure plenum with infinite paths of least resistance for outdoor air to be drawn in.

You may not be sucking it through the roof, but you mave have numerous openings through exterior walls above the ceiling and below the roof. Conduits for signs, building mounted lighting

Take a look at book page 113

http://www.masongrant.com/pdf_2008/ASHRAE_HCDG_C7_Mold.pdf

Sometimes moving air through the ceiling keeps humidity from building up, other times it causes it.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[willard3]

If your air system is at all complicated, air balance becomes almost impossible with ceiling plenums and conventional construction.

 

[texanman]

One other issue: although you mentioned roof insulation, and other components of the roof, the exact R values will be unknown. This could effect your heat load calcs, the plenum will be very hot.
regards,