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Old 03-23-2009, 09:04 AM   #1
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Duct sizing

A question that I've been wondering about. For (flex) duct carrying conditioned air in an attic... if you have a choice of two sizes that are legitimate according to professional standards, are there advantages to pushing the air speed up to the higher limit?

I am aware of ACCA Manual D and have the book. In it, they document how max airspeed should be 700 fpm. However, some pros on Hvac-Talk have told me it is not a super big deal to exceed that a little. I believe them, in part because my house duct system does that and the only loud noise came from cheapo plastic grilles. After decent metal grilles installed, no noise.

I'm assuming that the total duct backpressure is compatible with whatever blower is pushing it. In other words my question is for a system without problems of too much static pressure.

One example: Designing a duct system for a 400 cfm dehumidifier blower, it appears that a duct carrying approximately 200 cfm could be acceptable in either 8-inch or 10-inch size. The 10-inch would give velocity of 367 fpm and a pressure drop of .05 inch water column per 100 feet. The 8-inch choice would give 573 fpm and .14 pressure drop. Neither pressure drop seems enough to threaten the max blower ESP (external static pressure), unless lots of length or other fittings are involved. Does it matter which size is installed?

Other things being equal, it appears the smaller duct would have less energy loss due to smaller area -- assuming no excessive restriction of course. With some kinds of fan motors, I know that lower back pressure means lower energy consumption. In this example it appears the difference would be insignificant.

I'm such a newbie at this that I may well be overlooking something important. Please tell me if I am.

Thank you -- C44


Last edited by C44; 03-23-2009 at 09:23 AM. Reason: Remove specific 35-ft length, instead use PD/100ft.
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Old 03-23-2009, 10:15 AM   #2
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Add in the PD of the register.

Draw the run out on paper, as it will be actually ran, and add up for the bends.

May come out that a 9" is better then a 8"(9" is not common in many areas).

While true, that 10" has about 28% more surface area.
It may work better, then having an 8" at 2+" of static pressure.


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Old 03-23-2009, 11:26 AM   #3
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Amen to that

I would surely want to *always* keep fan ESP somewhere within the documented range. For a newer Thermastor that seems to be 0.4 inch w.c. The older Thermastors have lower limits. For me, a hazard is I might omit certain components in the paper model, when they will be there in the real world. Some experience on my part would be a big step forward.

As some have bragged on Hvac-Talk, the Aprilaire dehu range has always had much stronger fans and can withstand much higher ESP. They publish those numbers clearly. The tradeoff as I see it, is Thermastor shines in energy efficiency and Aprilaire generally has more capable fans. Each is outspoken about the areas where they excel.

Best wishes -- C44
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Old 03-23-2009, 04:24 PM   #4
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Well done C44
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