return cans as plenums?

[newbie]

I want to know what some other heating guys opinion are on this. I see ceiling return cans, they are referred to as b-boxes around here by the suppliers, being used as plenums right off of the ahu or furnace. They are essentially just 12 inch long square to rounds. i.e. 20 by 20 to 16 round by 12 inches long. Its my opinion that these would be bad for airflow to use right off the bat and turbulently noisy. I just made a new word ( turbulently).
I could see using one of these down stream aways but what do you guys think?

[device]

yeah i undestand whaty you mean..but if its commercial application..well the air dont care..

[newbie]

no its residential, but can you explain why the air doesnt care in commercial?

[Rob]

I want to know what some other heating guys opinion are on this. I see ceiling return cans, they are referred to as b-boxes around here by the suppliers, being used as plenums right off of the ahu or furnace. They are essentially just 12 inch long square to rounds. i.e. 20 by 20 to 16 round by 12 inches long. Its my opinion that these would be bad for airflow to use right off the bat and turbulently noisy. I just made a new word ( turbulently).
I could see using one of these down stream aways but what do you guys think?

Where do you see these 'return cans'?
Attic installs?

[cactassdupree]

The manufacture. Will tell you the size needed for both supply and return plenums. They size them so the unit will run to it's best. If you shorten them or take off to soon it will work but not without compromising the efficiency of the unit. It may put undo stress on the fan and shorten the life of it. A little larger is better and less noisy. (just an opinion) dupree :)

[newbie]

i see these installs in crawls and attics, they are used on both the supply returns, I think its cheezy but I talked to some people who do it and they think its the bees knees. I have never seen any documentation on a furnace that gave proper plenum sizes. They are square to rounds with a 1/2 down for plaster guard and 1 inch out for screw backing for the registers. they screw the 1/2 inch down to the supply and return then go with round the rest of the way .

[device]

i should think 20x20 does not equal the same area of air displacement.they are creating a pinch point right from the unit.So what in theory might be happening is the air flow out of the plenum is actually less than needed.the backedup air will become super reheated .Thus throwing out the whole system. let alone over working the fan and creating the air choke point.That was why plenum were created was so the air could actualy slow down @ this point while it has time to distribute to the runs..Just like the return air gets wider right before it re-enters the furnace.It needs to slow down so it can drop all the dust and crap on the filter.And even flow through the filter.
I have see furnaces where the return has come back and made a sharpe bend right before the unit(furnace) and the duct was not made larger,and no turning vanes were installed.
so what was heppening was the air was going aaround the corner on the ouside (heel) of the elbow and no slowing or being distributed by the vanes.
thus only the bottom 2 inches of the furnace filter was dirty.
The rest of it was spanking new!
so i quickly sold him a brade new furnace and that was that :)

no j/king , we made a new return and made it right..it was quieter and the air did not have a burn smell to it when it had been off a few hours.
funny..some peeps just dont get taught right.
as i was saying the air dont care..its the people that have to live there that do..
sorry its an old saying from the sheet metal trade..

[newbie]

thankyou guys, that was very in depth and comprehensive device , I appreciate it