Distance between Duct and Grills

[ReidnerVEC]

Is there a minimum distance between Grills and a duct, not connected to this grill, that needs to run over it? See the image below the situation.

 

[EnergyProfessional]

Obviously the grille needs to be attached to the duct. Otherwise the duct will draws air from the ceiling cavity. That cavity could communicate with the exterior, or other rooms or levels.

With your proposal one could even argue you turn that cavity into a return plenum, which would require all the necessary fire protection.

 

[ReidnerVEC]

The return register it's a return grill and the duct it's a supply duct. The intend of this question is coordination and the return air efficiency, if there are restrictions regarding the air flow if We have such obstruction above it.

 

[CodyK]

There are no code minimums that I am aware of. A quick Google came up with multiple sits listing 6" minimum in front of the return grille in a room, so above it wouldn't be any different. It also depends on the overall layout. However, if there isn't space to move the supply duct, than there isn't space.

To EnergyProfessional's point, make sure everything in the ceiling is plenum rated.

 

[EnergyProfessional]

I see. You have plenum return. Plenum return relies on very low pressure drop. So you want the return grille as large as possible and as unrestricted as possible. There isn't a simple equation, but larger grille and more distance is better. Also note that if you design to have 3" space, in real construction it may be less. A supply duct also may have insulation. It isn't clear if your sketch shows the correctly. i would jsut move the return grill away from the duct.

 

[MedicineEng]

If the duct covers the entire area of the return grill I would reckon that the it will block most of the flow, unless you have your return duct fairly close.
Return plenums can be tricky.
In the past we had to add some small circulation fans in big plenum return ceilings where the the return duct was located in a corner of the space, just to keep the air moving and increase the return on the areas further away from the return duct.

 

[GT-EGR]

If you want to put a number to it - you first need to determine how much air will come up through that grill. Then the gap between the grill top and the duct imagine as a really thin duct of its own. Say the grill is 24x24 and your duct sits 4” above, then you’ve got 4 slim ducts on each side of the grill 4”x24”. Take your return airflow, divide it by that combined area, and hopefully you are in the 100 fpm ballpark, probably 250 is fine, maybe even 500 fpm. Above that might get some noise and restriction on pressure in the room.

Also don’t forget your duct will be insulated and maybe some hangers/flanges, so you need space for that.