# Dead Vent: Linear Slot Diffuser Size?



## tjambro (Jan 16, 2007)

Hi Folks,

I have started on my room within a room basement build featuring 3 layers of rock and GG and I am at the planning stages for ventilation. See my signature for the build thread.

I plan on building soffits around the theater room as well as the sound lock room. Due to my low 7'3" ceiling, I only have a tad less than 6" above my doors so flex is out for the soffits. At this time I plan on fabricating my own ducts with ductboard and surrounding that with mdf.

I don't want to connect the theater to the house HVAC due to sound issues and am currently planning on building a Dead Vent setup.

Bryan Pape (bpape) did my sound analysis and he suggested the equivalent area of 8" flex (about 50 sq. inches) would be sufficient for the theater and sound lock. I was thinking about making the theater soffits closer to 60 or 70 sq. inches depending on how much sq. inches I could get out of the 4' wide duct board. I was thinking of making the soundlock soffits of a smaller area (about 28 sq. inches which is roughly equivalent to 6" flex) due to the room only being about 8' across.

I will most likely purchase two 8" or 10" Fantech inline fans as descibed on the Dead Vent page. One will be for the supply and one for the return and each will have a rheostat so I can adjust air flow. When the room is finished and sealed tight, I will most likely leave the fans on low just to keep the room from getting stale.

Here's how I see the layout as of now:










The blue lines are where I am thinking of running the supply lines and the red is for the return side. The blue and red rectangles are the supply and return registers.

I was thinking of going with a linear slot diffuser, also described on the Dead Vent page. Bryan suggested that I put the supply at the front of the room which makes sense but due to low ceiling height the screen will be up close to the ceiling and I planned on a curved soffit in the front with small can lights so the linear diffuser wouldn't work in that location.

Do you think my proposed diffuser location would be ok? How big of a diffuser should I buy? How many slots and how wide should the slots be? 

I know <250fpm is the desired output but I don't understand the specs I have seen and how to determine the right size. I will have a large return in the soffit back by the projector to remove heat from the pj. All equipment other than the pj will be in the soundlock area. I was planning on a 4" flex sized return directly over the equipment rack.

Any help would be appreciated.


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## Mike P. (Apr 6, 2007)

Moving air is all about balance. More air will follow the path of least resistance which is the shortest path and/or larger duct size. I would suggest dampers at all your "T" intersections so you can control how much air goes where. The goal is to have the proper amount of air to each room as well as same amount of air entering and exiting the rooms so you have a neutral air pressure. In your design the laundry room doors will have to remain open so that the return air can balance with the supply air.


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## tjambro (Jan 16, 2007)

Mike P. said:


> Moving air is all about balance. More air will follow the path of least resistance which is the shortest path and/or larger duct size. I would suggest dampers at all your "T" intersections so you can control how much air goes where. The goal is to have the proper amount of air to each room as well as same amount of air entering and exiting the rooms so you have a neutral air pressure. In your design the laundry room doors will have to remain open so that the return air can balance with the supply air.


Thanks Mike, the dampers are a good idea but more likely I will close or block a portion of a vent if need be since that will be easier for me as they will be readily accessible. Good point about the laundry room doors. I'll have to see if I will have to leave them open or if the free space under the door will be enough.


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## Ted White (May 4, 2009)

Tom,

You wouldn't need more than a 6" fan. One in supply and one in return will move a great deal of air


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