Return grill ducting and AHU

[geopat69]

Hi all.

I am reviewing some office AC ducting drawings and had a query regarding the return grille that are typically shown in plan.

The office is a mulitstory building with drop-in ceiling tiles.

On the plans, i see all the supply air ducts and the direction of airflow feom the AHU. But there are no ducting shown for the return grilles back to the AHU. To me this indicates the return air is discharging into the ceiling cavity. Is this commonly done in Australia?? If so, does that mean the AHU is pulling 100% outdoor air? (Or alternativly is the return grille ducting not shown on plans for ease of reading and diagramtic reasons)

 

[Snickster]

Return air typically flows through hallways or the cavity above suspended ceilings back to the HVAC unit when the unit is located in a mechancal room remote from the location of the spaces it serves. This saves on cost of ductwork and fan power that is required to overcome return duct friction losses. Transfer grilles are installed in the walls of the rooms connected to the hallways to allow the air to flow. However when the flow is low enough the doors are undercut with about an inch or two removed to allow the air to flow into the hallway under the door instead of using transfer grilles. In suspended ceilings transfer grills are installed in the walls above the ceilings that allow the air to flow all the way back to the equipment room. These openings usually have fire and/or smoke dampers installed. 100% outside fresh air is never used in an office space but spaces like laboratories.

 

[nuuvox000]

In the USA, this is typical for office buildings. Return air plenum above the ceilings (no duct) and then there will be a main return duct opening that takes it back to the AHU. I don't think it would be doing 100% outdoor air.

 

[geopat69]

Ah yes. I see what u mean. And yes there is a designated separate ahu plant room to one side of the plan. It appeqrs as though this office has the plenum in the ceiling.

Thankyou that really helps.

** out of curiosity...would there 'eventually' be a short run of ducting from the ceiling plenum to feed directly into the ahu return air?? OR would the air plenum just discharge into the whole AHU plant room??

 

[nuuvox000]

I've seen it done both ways, with a short piece of duct into the office ceilings or the entire mechanical room could be part of the plenum as well. In my opinion, it's difficult to make the entire room a plenum because everything in the plenum needs to be plenum rated and that's very hard to do in a mechanical room; at least that's the ICC code.

 

[trashcanman]

Snickster: If the hallway is an exit corridor, NFPA prohibits its use as a return air path. A fire in any room served by the AHU would flood the exit corridor with smoke.