cold air return sharp corners?

[tobra]

How important is it to have rounded corners( 90 degree bends) in the cold air return? I understand on the supply side of the furnace that you what the air to flow as smooth as possible, is this also the same for the return side? My return plenum and trunk are basically rectangle boxes, all the corners are square. Would this be a possible source of noise. I am getting a fair amount of noise for my cold air returns and I know part of the problem is the grilles are undersized. I will be making them larger and use better grilles( I have one grille that whistles). But is having the square corners in the plenum adding to the problem? It would take a lot more work to change, but will do if I need to.
System : Carrier 58mvc 80,000 btu upflow in basement
Cold air plenum is attached to the side,bottom 12x23 square corner
Runs up to floor joist 8ft., floor joist are boxed in square corner. Runs though floor joist for 6ft then down into 16x 8 trunk and up into a return in the wall( this is the one that whistles, also hear burner and clicking of gas valve), again square corner. Trunk extend 15ft to 2nd return.
Have added 8x16 return in basement off furnace side of plenum( basement did not have return). Did not seem to quiet the return on main floor. So should I remove the current return line and reinstall with rounded corners(keeping flow in mind as I do it) as well as install larger grilles?

 

[Hube]

Properly sized RETURN ducts/passages are usually sized slightly higher in area than supply ducts/runs.(10-15 %) This "oversizing" will allow for unstreamlined conditions in the returning of air coming back to the blower. Therefore, ragged edges, abrupt turns, etc in return passages will have NO adverse effect on the SUPPLY part of the air system at all.
Since you say the return grilles are undersized, that is more than likely the cause of your noise problem. Also, even with a properly sized system I have always installed an inner 'acoustic' liner to approx 5 ft. of the main return duct before it enters the blower compartment.Obviously, the duct has to be enlarged to allow for the lining thickness.
Duct and grille sizing is extemely important. That 16x8 return duct is 128 sq. inches in area ,but a grille size serving an 16x8 duct should be approx 25% larger in area depending on the design of the grille. Some grilles have more restrictions than others.
And note that a joist space (7.5" deep x 14.5") has only an area of approx 108 sq inches and it is being used to carry that 16x8 duct (128")
Math is very important when sizing ductwork.

 

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[Gator37]

elbows

Talking residential units here....Gas heating electric cooling furnaces do not have the same static pressure capability (for pushing air thru the duct system) as a straight cool unit with electric heat or a heat pump with aux elect. heat because the blower has to overcome the pressure drop of the heat exchanger in the furnace which does not leave much for external static.

Duct sizing is a little more liberal for gas units or should be when designing. As indicated in the previous response above, the return square elbows should not be a concern if original sizing was based in the use of a gas unit, unless you have numerous elbows which is normally not the case. If you think there is a problem you can check you blower motor and see it if has electric terminal connections for high, medium or low speed and change the terminals to high speed if applicable. You can also check the CFM of air delivered to each room, add it up and check to see if the total is close to the unit rated CFM of the unit.

For what it is worth on simple systems I size for .05" per 100' of duct for furnaces and .08" to .1" per 100' of duct or other than furnaces. I also select a furnace with a little more capability from the manufacturer's charts. Different people have different approaches