I just did a little experiment to determine how much air return my 5HP DC really wants.
The test was run with single 6" duct open (jointer). The 5HP Oneida Dust Gorilla Pro has in 8" inlet, which transitions to 7" after about 10', which transitions to 6" after 10 more feet, then there is about a 20' 6" run to the jointer:
- turned on the DC in its 7'*6' closet
- slowly closed the heavy exterior door until I felt pressure
- let the pressure in the closet naturally reopen the door.
Not perfectly scientific, but perhaps adequate to ensure there was no back pressure. I repeated this half a dozen times, sometime closing just little more than where the door opened previously, and got basically the same result.
My 8' door wants to be open 5". If I push it closed to 4.5" it goes back to 5" and holds. 5*96=480 sq inches, plus a bit more for the pie slice at the top and bottom of the door. Add to that my existing 10" duct, which is probably 8" in the bends, and we have something around 550 sq inches.
When I opened up 4 blast gates (a combination of 6", 4", and 3") I could hear the DC reving up as I opened each blast gate, the closet door wants to open 7.75 inches, which is 744 square inches of return.
That's a lot more than the 153 sq inches given by a perfect 14" duct.
Unfortunately, my DC is along an outside wall and there isn't very much room in the attic to work considering the roof pitch.
This makes me curious how much return area others have, and if they actually verified there was no back pressure.
Frank's Pratts 10*30 rigid duct gives 300 sq inches (- area of insulation). Frank does your closet door close without any pressure when the DC is sucking without any signs back pressure?
Am I missing something in this experimental design?
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Originally Posted by mark mcfarlane
