Downdraft Vent Ducting - Qs About Duct Size And Transitions - HVAC - Page 2 - DIY Chatroom Home Improvement Forum

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Old 12-28-2016, 07:35 PM   #16
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Re: Downdraft vent ducting - Qs about duct size and transitions

Originally Posted by user_12345a View Post
if the op wants a specific flow rate would have to go through that procedure regardless.

Not exactly. The fan ability is picked to meet the exhaust duct for safety.

the grease factor i didn't think about but it makes sense in the same way that if dryer ducts are sized too large, the velocity isn't high enough to vent the lint.

if there's a risk of having grease buildup there would have to be a minimum velocity to size to, and if the fan is inadequate for the required cfm, get a more powerful fan.

180cfm through 4" is a lot especially if the pipe is long and/or or flex, most hood instructions spec 6".

Just requires a fan to be rated to work against a 2" ESP. The longer the run is, the velocity you need.

main point was that the ratings are totally deceptive and calculations are needed to see how it will perform in the field.

they shouldn't be allowed to sell fans with cfm ratings at 0" static. imagine if they sold furnaces like that.
The .5" ESP that most furnaces are rated at. Is just as much of a joke as 0" for an exhaust fan.

There are those applications that the make up supplies enough air to bring the static down to virtually 0". However, that is something that shouldn't be done 99.95% of the time.
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