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08-23-2009, 09:30 AM   #1
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## Ducting question

One particular run (2nd floor) is long and tortuous.

Its takeoff is located close to the basement furnace. There is a section of 6" duct (accessible) which runs about 20 feet, then elbows up into the structure (not accessible).

Would increasing the size of the 20 foot section change the dynamic of the airflow (CFM/velocity) into that room?

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08-23-2009, 11:16 AM   #2
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http://www.engineeringtoolbox.com/du...oss-d_444.html
gives you duct friction but you also need the 'fan curve' for your HVAC system to solve for the end result CFM and FPM.

BTW, a low-tech way of measuring CFM is to tape a huge garbage bag over the register and time how long it takes to fill it.
For approximate velocity in feet/minute, divide the CFM by the duct area [0.2 ft² for a 6" round duct, so 100 CFM would equal 500 FPM].

You could also try a variable speed duct booster fan, but it also boosts the noise level so put the fan at the takeoff end. Comfort vs. fan noise is up to each person so with variable speed you can select your own optimum point.

Last edited by Yoyizit; 08-23-2009 at 03:57 PM.

 08-23-2009, 07:13 PM #3 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,588 Rewards Points: 6,140 Yes it would. How much depends. You may also need to increasethe amount of return you have.
08-24-2009, 06:20 AM   #4
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Quote:
 Originally Posted by beenthere Yes it would. How much depends. You may also need to increase the amount of return you have.
Been, would this work with an 'increaser' you mentioned in the 'other' forum?

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 08-24-2009, 06:25 AM #5 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,588 Rewards Points: 6,140 Best if you use a take off the size the new duct is, from the main duct/plenum, and then use a reducer at the end that goes into the unaccessable area.
08-24-2009, 06:52 AM   #6
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Quote:
 Originally Posted by beenthere Best if you use a take off the size the new duct is, from the main duct/plenum, and then use a reducer at the end that goes into the unaccessable area.
I thought that your answer would be something along that (above) line LOL.

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