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Old 2008-07-15, 11:59pm
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Hayley Hayley is offline
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This was posted by Mike Aurelius:

http://www.lampworketc.com/forums/sh...90&postcount=8
Quote:
Originally Posted by MikeAurelius View Post
Ok -- your problem is manyfold --
2) You are reducing the size of the duct right at the hood . . . . This causes severe reduction in airflow. Ducts should never be reduced in size in the direction of the airflow, they should always increase in size if necessary.
The person started this thread had the same GlassCraft hood as I do and used a reducer to change from the 10" fan to 8" ducting . . . comments from Mike Aurelius and co.:
http://www.lampworketc.com/forums/ne...reply&p=861958
Quote:
Originally Posted by MikeAurelius View Post
Here are the issues that are causing this system to not work, besides the CFM issue:

2. The duct absolutely has to be the same diameter all the way through the system. Reducing the size of the duct from 10" to 8" reduces the duct area by slightly more than 28" square inches. This causes huge amounts of back pressure.
http://www.lampworketc.com/forums/sh...3&postcount=12

Quote:
Originally Posted by MikeAurelius View Post
As far as your question about duct work sizing - are you asking if it is ok to (for example) run 8" to the fan, then after the fan run 10"? The quick and dirty answer is yes. You can always INCREASE the size of the ducting in the direction of the airflow, but you should never DECREASE the size of the ducting in the direction of the air flow. (caps for emphasis).
From bhhco:
http://www.lampworketc.com/forums/sh...51&postcount=8
Quote:
Originally Posted by bhhco View Post
Reducing the vent pipe from 10" to 8", essentially makes it a funnel, and results in a 33% surface area reduction... and thus significantly reduces the air flow. . . .
ETA: This is what Dale said in Leslie's thread – if one's actually fan size is in line with the smaller ducting AND the duct run is short, AND a test of your system proves that it's working effectively, then it's fine.

http://www.lampworketc.com/forums/sh...10&postcount=6
Quote:
Originally Posted by Dale M. View Post
Fans have a outside diameter that are dictated by the construction methods of the fan, this is not necessarily the size of the duct required for fan to operates efficiently.... Fans operate against what is called "static pressure" or resistance to flow... Long duct runs have more static pressure that short runs, small diameter duct runs have more static pressure than larger diameters duct runs.... In the end, if you have fan that is operating efficiently against low static pressures and a test of your ventilation proves it is working effectively you are good....

Dale
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Last edited by Hayley; 2008-07-16 at 9:10am. Reason: edited to add
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