ceiling return 1

[radwin]

is it advisable to have ceiling return at the roof with insulation? Because I imagine it also have heat load.

 

[willard3]

I don't understand the question...........

 

[imok2]

Yes

 

[radwin]

I mean ceiling return with a roof above the ceiling. most ceiling return with a drop ceiling that I see are between floors not below the roof. is it because the space below the roof and above the ceiling has more heat load?

 

[Drazen]

Plenums are normally installed having space/architectural limitations in mind, and roof would need to be insulated.

In thermal load calculations, additional outside heating/cooling load has to be taken into account, and that is about it.

 

[Waramanga]

Not sure about other people but I can t say that I have seen too many plenum return systems with a roof as the upper part of the plenum. Sealing the roof is a pain and badly done, worse still something which mech engineers have little control over! Anyone with a good experience with this arangement???

 

[Ziggypump]

Insulate the return duct as well.

 

[MechEngNCPE]

I just did an office top floor and had to deal with this. Just keep in mind that your RA temp will be higher than normal, and factor that in to the mixed air temperature when doing your load calcs. Find the U-value of the roof and model the system either by hand or with HAP or Trace

 

[DRWeig]

I've seen it a few times, including the last building in which I was employed.

Do what MechEngNCPE says, he's on the money.

Good on ya,

Goober Dave

 

[317069]

-it is a good idea to insulat plenum, not because it has a separate load but because a more insulation will reduce your total load.
(load calculation will cover the space from the floor to the roof not to the false ceiling)

- return plenum has no separate load than building load, it is a part of building load calculation, it effects on load distribution between zone level and coil level.

- example: let say we have a zon in building has a roof load of 50000 Btu/hr.
- in case of return duct, all this 50000 btu/hr is a zone load which determin the cfm required for this zon.
- in case of return plenum, a part of this 50000 Btu/hr will go directly to the unit's coil and the rest will go into the zone.

- in both cases the coil load is the same but the zone load is different.

 

[cry22]

317069 has it right:
If you run two calcs in Trace or HAP with and without RA pleneum, you will see a different air flow for the zone as indicated by 317069 - the RA plenum requires less CFM for each zone/space than the ducted return. substantially less - as nuch as 30% I've seen.

In cold climates, it is a good idea to have RA plenum to keep pipes in the plenum from freezing.

the thing to watch for is a conversion of a RA plenum system into a Ducted system. Something not to do.

 

[radwin]

Thank you for all the replies. I got a lot of insights. I appreciate all of your time. Thanks!