Spacing of Top TakeOffs and Turbulance

[Div Grad]

I'm having a big reno done and part of that involves ducting to new rooms. I don't have a problem with how I see new supply ducts being added but I have a question about how the cold air returns are being done.

Top take-offs are being added to the main cold air return in the basement in order to service the new rooms. These returns then run along the joist space before heading up to the other floors. Some of the new take offs are being added in joist spaces that already have an existing take off. Because the joist space is narrow having two take offs in one joist space makes them almost touch where they enter the main cold air return. Does having the takeoffs close together cause any major problems with flow / balance / turbulance? I say major because I can visualize other ways this could be done but it'd involve at least one 90 degree turn and I understand they cause their own problems.

I also don't want to try to get the HVAC person to do this differently if no real savings result from it.

[tech45]

I'm not aware of any rule of required spacing of take - offs. I have cut plenty of them side by side and still had flow where I wanted.

Just remember that top take offs are a 40 ft equivilant length in most cases. Not sure how much air your expecting from this return ( 200 cfm or so at 8 inch ).

Another option would be to pan the joist. This works best with an oversized opening cut into the trunk duct serving the joist space ( acting as a low loss change in direction @ 10 equivilant ft ). This will get you about 500 cfm ( 14 by 9 available space assumed ) at a really low s.p. The existing duct in this area will take away some from that figure , it should be a sheet metal runout ( not flex ). But I'm sure it is...no one runs flex in basements , right : )

edit : that panning suggestion is only for return air , not supply air. If your guy is running individual returns to each room he is doing a good job , let him keep working ( after re reading your post ).

[motoguy128]

If you have adequate return air, the velocity should be realtively low, so turbulence and other issues that affect supply air, shouldn't be issues for return air.

The biggest thing with returns is that they are sealed. Especially if panning joists and even more improtant if the basement is not finished or an unconditioned space.

[beenthere]

It will have some effect.
How much effect depends on how big your trunk line is or isn't.