Friction rate and velocity, the two levers
Every foot of straight duct costs the air a little static pressure as it drags along the wall. Expressed per unit length, that cost is the friction rate: inches of water gauge per 100 feet of duct in imperial units, or pascals per metre in SI. Velocity is how fast the air is travelling through the section, in feet per minute or metres per second. The two are linked. Push a fixed airflow through a smaller duct and the velocity goes up, and so does the friction rate; open the duct up and both fall.
Sizing a duct is choosing where to sit on that trade-off. A bigger duct is quieter, loses less pressure and saves fan energy for the life of the building, but it costs more in steel and takes more space in the ceiling. A smaller duct is cheaper and tighter to route, but it is louder and makes the fan work harder every hour it runs. Friction rate and velocity are the two dials that set the balance.
The friction chart and the equal-friction method
The classic tool is the duct friction chart. It ties together four quantities for round duct carrying standard air: airflow in cfm, duct diameter, velocity, and friction rate. Fix any two and you can read the other two straight off the chart. As a worked feel for the numbers, around 6000 cfm in a 28 in. round duct runs at roughly 1400 fpm and loses about 0.08 in. w.g. per 100 ft. The chart is a graphical solution of the Darcy-Weisbach friction equation, which is why friction loss climbs so steeply with speed: it rises roughly with the square of velocity.
The most common way to use the chart on a low-pressure commercial system is the equal-friction method. You pick one friction rate and size every section of duct to it. Because the airflow drops as the run branches out while the friction rate stays constant, the velocity falls on its own toward the ends of the system, which quietly reduces air noise near the outlets. A friction rate around 0.08 to 0.10 in. w.g. per 100 ft is a typical starting point, often near 0.10 for supply and 0.08 for return. That is a design guideline, not a code limit. The sensible band runs from about 0.05 in. w.g. per 100 ft for a quiet, energy-efficient system up to about 0.15 for a tighter, cheaper one, and the right value is always project specific.
Equal friction is popular because it is simple and it suits constant-volume systems well. Its main weakness is that it does not equalise the pressure available at each branch unless the layout happens to be symmetrical, so balancing dampers are needed to trim the short runs near the fan against the long runs out to the far rooms.
Recommended air velocities
Velocity is capped from above by noise and cost, not by physics, so the useful figures are recommendations rather than limits. The long-standing comfort-air-conditioning velocities, by building type, are:
| Duct |
Residences |
Schools, offices, public |
Industrial |
| Main duct | 700-900 fpm | 1000-1300 fpm | 1200-1800 fpm |
| Branch duct | 500-700 fpm | 600-900 fpm | 800-1000 fpm |
These are recommended design guidelines, not code figures. A rule of thumb sizes branch ducts near 80 percent of the main-duct velocity and the final runout to the diffuser near 50 percent, which keeps the air slow and quiet where it enters the room. Supply diffusers and return grilles are usually kept to a few hundred fpm face velocity for the same reason.
Velocity, noise and fan power
Velocity's penalties scale badly. Friction loss climbs with roughly the square of velocity, and the fan power to overcome it with the cube, so doubling velocity at the same airflow quadruples the friction and octuples the fan power. Higher velocity also generates more air noise directly, which is why the velocity is held down to keep the NC or RC noise level acceptable in occupied spaces. A rectangular main duct running through an occupied space is often kept near 2000 fpm for a relaxed noise target and below 1000 fpm for a strict one; round duct tolerates higher velocities before it gets noisy.
By velocity, systems are loosely grouped as low velocity up to around 2000 fpm, medium from about 2000 to 2500 fpm, and high above roughly 2500 fpm. The dividing line is not a single fixed number; it has been placed variously between 1500 and 2500 fpm depending on the source.
When equal friction is not the method
Equal friction is the default for low-pressure work, but two other methods appear on larger or higher-velocity systems.
- Static regain sizes the duct so the velocity is stepped down along the run, letting the recovered velocity pressure make up for the friction loss in the next section. The result is roughly uniform static pressure at every branch, which makes outlet selection and balancing easy. It is the usual choice for large, high-velocity and VAV distribution, most often in the high-pressure duct between the air handler and the VAV boxes.
- Velocity reduction simply assigns progressively lower velocities to the main, the branches and the runouts from a velocity table, then sizes each section from area equals airflow divided by velocity. It is quick but leans heavily on the designer's judgement.
Round duct and the equivalent diameter
Duct shape changes the friction before any method is applied. For a given cross-sectional area a round duct has the smallest perimeter, so the least wall for the air to rub against, which gives it the lowest friction loss of any shape. The advantage grows as a rectangular duct is squashed into a more elongated aspect ratio. That is why round and spiral duct is the efficient choice on long, high-flow runs where friction and fan energy matter, and it is one reason a spiral tubeformer earns its place alongside a rectangular line rather than instead of it.
To compare the two fairly you use the equivalent diameter: the diameter of a round duct that would carry the same airflow at the same friction loss as a given rectangular duct. It is the correct basis for reading a rectangular size off a round friction chart. See our round versus rectangular duct comparison for the full trade-off, and the rectangular and spiral sizing charts for the dimensions.
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From friction to the fan to the shop floor
Friction rate and velocity are not just a sizing exercise. They set the fan. The fan has to produce enough static pressure to overcome every loss along the critical path, the highest-resistance route from the fan to the most remote outlet, plus the external static of the coils, filters and terminals. That total static pressure is what the fan is selected for, and it is also what fixes the duct pressure class. A higher system static pushes the section nearest the fan into a higher pressure class, which drives the SMACNA construction the shop builds to: heavier gauge, closer reinforcement, a stronger joint and a tighter seal class. Size the duct too aggressively to save steel and you can push the pressure class, and the fan energy, up more than you saved. The sizing decision and the fabrication cost are two ends of the same calculation.
Specification language you will see
A mechanical specification usually fixes the sizing method and a velocity or friction ceiling rather than every duct size: "Low-pressure supply ductwork shall be sized by the equal-friction method at a friction rate not exceeding 0.10 in. w.g. per 100 ft, with main-duct velocity not exceeding 1300 fpm in occupied areas." A VAV spec instead reads: "High-pressure ductwork between the air handler and VAV terminals shall be sized by static regain." Either way, the spec is setting the trade-off between duct size, noise and fan energy that the rest of the design then follows.
FAQ
What is duct friction loss?
It is the static pressure the air loses rubbing along a straight duct wall, stated as a friction rate in in. w.g. per 100 ft (or Pa/m). The friction chart ties airflow, diameter, velocity and friction rate together so any two give the other two.
What is a good friction rate for duct design?
Around 0.08 to 0.10 in. w.g. per 100 ft is a common equal-friction design value, often 0.10 supply and 0.08 return. It is a guideline, not a code limit; the sensible range is about 0.05 for quiet, efficient duct up to about 0.15 for tighter, cheaper duct, and the right number is project specific.
What are recommended HVAC duct air velocities?
Recommended main-duct velocities run about 700 to 900 fpm in residences, 1000 to 1300 fpm in commercial buildings and 1200 to 1800 fpm in industrial. Branches are lower. A rule of thumb sizes branches near 80 percent and the runout near 50 percent of the main-duct velocity. These are guidelines, not code.
Why does duct velocity matter for noise?
Friction rises with roughly velocity squared and fan power with velocity cubed, so doubling velocity quadruples friction and octuples fan power. Higher velocity also makes more air noise, so velocity is capped to hold the NC or RC noise level in occupied spaces.
Does round duct have less friction than rectangular?
Yes. A round duct has the smallest perimeter for a given area, so the least wall friction and the lowest loss of any shape, and the gap widens as a rectangular duct gets more elongated. The equivalent diameter is the round size that matches a rectangular duct's airflow and friction.
