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 10:23 AM.
Reason: Remove specific 35-ft length, instead use PD/100ft.