Friction Rate
The friction rate tells us how much friction our ductwork should produce, per 100’, so that we consume the rest of our available static pressure (ASP) budget with our specific layout. The idea here is that we’re working backwards so that we can land on our selected ASP when all is said and done.
If you haven’t read the post on ASP, you can do so here. Most HVAC contractors are familiar with seeing the Friction Rate on their duct calculators, like the one shown here. Many have been told that they should always use 0.10 IWC (inches of water column) as their design rate, as if it’s something that can be arbitrarily selected. The truth of the matter is that the layout, materials, and fittings of the duct system, along with the specifications of the blower that will be used, is what determines the friction rate.
[box type=”tick” icon=”none”]A good analogy is to think about making a cool drink in the summertime. Your glass only has so much capacity (ASP). First you pour in the liquid of your choice (Components), and whatever room is left over (ASPnet) allows you to put in the ice cubes (Ductwork) that allow your drink to maintain the ideal temperature. If you put too much liquid in, you didn’t leave yourself any room for the important stuff – the ice cubes that keep your drink cool. The Friction Rate allows you to tell whether or not you’re going to need to remove some of the liquid, or perhaps get a larger glass, prior to dropping in the ice cubes.
The relationship between ASP and duct size is represented in the following equation, via the Friction Rate (FR):
NOTE: ASPnet = ASP – Component Static Pressure Drop
Thus, it’s what’s truly “available” for the ductwork itself.
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FR = (ASPnet x 100) / Total Effective Length
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The thing to keep in mind, at this point, is that the friction rate is what we use to select the appropriate duct size. The smaller the FR, the larger the duct. According to Manual-D, we should end up with a friction rate between 0.06-0.18 IWC. If we’re outside of this range, we should make some adjustments to our design.
To increase the FR, one must either increase ASP or decrease Total Effective Length (TEL). You’ll rarely have a friction rate over 0.18 IWC. But, if you do, you’ll need to select a blower (or blower speed) that will reduce the ASP.