What Friction Rate Should I Use?
The friction rate is how much pressure the duct eats per 100 ft. Pick it from the static pressure you have to spend, not from habit.
What it is
Friction rate is the pressure drop in inches of water gauge for every 100 ft of straight duct. A lower rate means a bigger duct, slower air and less noise. A higher rate means a smaller duct, faster air and more pressure used up. Sizing every run to the same friction rate is called equal friction sizing, and it is what the duct size calculator does.
The Manual D way
ACCA Manual D works the design friction rate out from your own job:
- Start with the blower’s external static pressure from the manufacturer’s data, at the airflow you are running.
- Take away what the other parts use: coil, filter, grilles and registers, dampers and anything else in the air path.
- What is left is the available static pressure for the ducts.
- Find the total effective length of the longest run: straight duct plus the equivalent length of every fitting.
- Design friction rate = available static pressure × 100 ÷ total effective length.
Example: if 0.12 in. w.g. is left for the supply duct and the longest run is 150 ft effective, then 0.12 × 100 ÷ 150 = 0.08 in. w.g. per 100 ft. The numbers are for illustration only. Use your own.
The common rule of thumb
When you do not have a full design, many residential branch runs are sized somewhere around 0.08 to 0.10 in. w.g. per 100 ft. That is a common rule of thumb, not a code requirement, and it is why 0.08 is the default in the calculator. If there is a Manual D report or an engineer’s drawing for the job, it overrides the rule of thumb.
What changes when you move it
Here is a galvanized round duct sized at each of the calculator’s presets. Diameters are exact, and velocity is in that diameter.
| Friction rate (in. w.g./100 ft) | 400 CFM | 1,000 CFM | ||
|---|---|---|---|---|
| Diameter | Velocity | Diameter | Velocity | |
| 0.05 | 11.2 in. | 581 fpm | 15.8 in. | 730 fpm |
| 0.08 | 10.2 in. | 703 fpm | 14.4 in. | 883 fpm |
| 0.10 | 9.8 in. | 769 fpm | 13.8 in. | 966 fpm |
| 0.15 | 9.0 in. | 906 fpm | 12.7 in. | 1,137 fpm |
Dropping from 0.10 to 0.05 adds roughly 1.5 to 2 in. to the duct and takes about a quarter off the velocity. Going up to 0.15 takes about an inch off the diameter and pushes the velocity up by roughly 18 percent, so the duct is likely to be louder and every fitting costs more.
Duct type changes the answer
The same friction rate gives a bigger diameter for rougher duct. Galvanized sheet metal uses a roughness of 0.0005 ft. Fiberglass duct board is rated at 0.003 ft, and fully stretched flex also at 0.003 ft. Flex that is compressed or sagging loses more again: the LBNL lab data give roughly 1.7 to 2 times the friction at 4% compression and 3.4 to 4.8 times at 15%, depending on size. Run the numbers in the flex duct calculator before you pick a flex size.
Check the velocity
After you pick a rate, look at the velocity. Above about 1,200 fpm noise and friction climb fast, and fitting equivalent lengths are only approximate below that speed. If a size comes out too fast, drop the friction rate or go up a size. The duct velocity calculator checks any size you already have.
Limits
Results can differ from another chart or the manufacturer’s data by 5 percent or more, and real ducts vary with joints, fittings and installation. This is a sizing aid. A Manual D and J calculation or an engineer’s design comes first, along with the manufacturer’s data and local code.
Next: size a duct, print the duct size chart, or read how many CFM per ton to get the airflow first.