Question about Jenn-Air Ventilation System for Custom Hood Installation, JXT7036A

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What size duct do I need

Need to moove 1500 CFM of Air over a distance of 75ft with four 90's before fan. There is another 20ft with two 90's after the fan. What size duct Is needed? Ducting is spiral, galvanized steel.
Currently ducting is 12" with a 3/4hp tube fan. I feel duct and fan are just too small to move this amount of needed exhaust that distance.
Are there online calculators for this? Thanks, Ryan

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There are online formula to calculate what you want
google ---formula to calculate air flow in pipes--
calculation will have to allow for the turbulence and restriction caused by the spiral construction of the tubing and the bends involved so you may have to incorporate several formulas to get the desired result
distance is critical as there are frictional coefficients to be considered as well
I would consider from large shop air circulation systems (as you see in Aldi, woolworths , sears etc ) that a staged diameter difference from motor to exhaust point be considered as that will keep a steady air flow for the length of the tube and a hp rating of 3hp 3 phase as a minimum
Talk with the engineers of shop and factory air conditioner designers and installers to get a close idea to what would be suitable for you

Posted on Aug 03, 2017


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Posted on Jan 02, 2017

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SOURCE: Jenn-Air Downdraft ducting roof Rack w/flapper

Hi Snowflow,

I'm Harvey the Master Plumber.

Your problem is: You are looking for a high quality roof mounted termination vent cap for 8" O.D. round ducting for your Jenn-Air downdraft Stove Top/Convection oven. The reason for the 8" ducting is that you are using insulated stove piping out of the wall of the house into a cold porch then up through the roof. You have come across a Broan Vent Cap that will accommodate 3-1/2" by 10" rectangular ducting and/or 8" O.D. round ducting. The problem with this Broan unit is it has an aluminum flapper that has no counter resistance mechanism to keep from readily flapping open and shut during weather related wind events. Is there something better out there? You found us through a Jenn-Air link. Help!

An 8" B-vent cap should work fine. Can be purchased at any plumbing supply. Just make sure you attach it with screws.

Feel free contact me again!
Please give me a rating here at before you sign off
Thank you,
Harvey your Master Plumber

Posted on Oct 14, 2008

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SOURCE: Jenn-Air Downdraft ducting roof Rack w/flapper

go to
there are many good high quality vents on the site

Posted on May 26, 2009

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