Ducts & air
Duct sizing basics: friction rate, velocity and equivalent length
A duct is sized from three numbers, the airflow it carries, the pressure the blower can spare, and the speed the air can travel quietly. Here is how they fit together.
What a duct has to do
A forced-air system needs about 400 cubic feet per minute of air for each ton of cooling, so a 3-ton system moves 1,200 CFM. The ducts have to carry that air from the blower to every room and back again, using only the pressure the blower can spare after the coil, filter and other parts have taken theirs. Ducts that are too small starve the system of air; ducts that are too big cost more, take more space and deliver air too slowly to mix the room.
ACCA Manual D is the industry method. The calculators on this site follow its logic in a simplified form: work out how much pressure is available, spread it over the length of the longest run, and size each duct to carry its airflow at that friction rate without exceeding a velocity limit.
Step one: the available pressure
The blower’s rated external static pressure, often 0.5 inches of water column for a standard air handler and up to 0.8 or 1.0 for a variable-speed unit, has to cover everything outside the blower. The coil, filter and accessories such as a humidifier or a zone damper take their share first:
A 0.5 in wc blower with a coil at 0.20, a 1-inch filter at 0.10 and 0.06 of accessories leaves 0.14 in wc for the ducts, supply and return together. The filter is the part people forget, and a restrictive filter can take a large share, which the guide to MERV filters and airflow explains.
Step two: equivalent length
Air loses pressure along every foot of straight duct, and much more in every elbow, takeoff and boot. Manual D expresses each fitting as an equivalent length of straight duct with the same loss. The duct equivalent length calculator uses these typical values:
| Fitting | Equivalent length |
|---|---|
| 90° elbow, metal | 15 ft |
| 90° elbow, flex | 30 ft |
| 45° elbow | 10 ft |
| Trunk takeoff | 35 ft |
| Register boot | 35 ft |
| Return grille box | 30 ft |
Flex duct is counted at 1.5 times its straight length, because its corrugated lining adds friction, more so when it sags or is pulled short.
The friction rate for a metal system
40 ft of supply and 20 ft of return in metal, three 90° elbows, one 45°, a takeoff, a boot and a grille box, with 0.14 in wc available.
- Straight: 40 + 20 = 60 ft
- Fittings: 3 × 15 + 10 + 35 + 35 + 30 = 155 ft
- Total equivalent length: 215 ft
- Design friction rate: 0.14 ÷ 215 × 100 = 0.065 in wc per 100 ft
The fittings are almost three quarters of the length. That is typical, and it is why a run with fewer elbows can use smaller duct. The same layout in flex on a 0.8 in wc variable-speed blower comes to 368 ft but has 0.49 in wc available, a friction rate of 0.133.
Step three: size each duct
With the friction rate set, each duct is sized for its airflow. The duct size calculator uses the ASHRAE friction relation for galvanized round duct and checks the result against a velocity limit:
A 400 CFM duct
400 CFM at 0.10 in wc per 100 ft, supply trunk limit 900 FPM.
- Size for friction: 9.83 in; size for velocity: 9.03 in
- Friction governs, so 10 in round
- In a 10 in duct: 733 FPM and 0.092 in wc per 100 ft
- Rectangular equivalents: 6 × 16, 8 × 12, or 10 × 10 in
Velocity limits
Air moving too fast in a duct makes noise, especially at registers and returns. Typical residential limits are 900 FPM in supply trunks, 700 in branches, 600 in return ducts and 500 at the face of a grille. The duct velocity calculator checks a duct against them. A 10 in round duct at 400 CFM runs at 733 FPM, inside the trunk limit; at 900 FPM it could carry 491 CFM.
Round and rectangular
Round duct carries the most air for its metal and has the lowest friction. Rectangular duct fits between joists and in low spaces. A rectangular duct is compared with round by its equivalent diameter, the round size with the same friction at the same airflow:
The flatter the rectangle, the worse it does. A 10 × 24 in trunk is equivalent to a 16.5 in round, although its area is larger than a 16 in round’s. Keep aspect ratios below about 4 to 1 where you can. The duct size chart lists common round and rectangular sizes with their capacities.
Rooms, registers and returns
Once the trunks are sized, each room’s branch carries that room’s share of the airflow, which the CFM per room calculator estimates from floor area and exposure, and ends at a register, sized with the supply register size calculator. The air must come back too: returns are often the most undersized part of a system, which the guide to why return air matters covers.
The duct you cannot see
Sizing assumes the air stays in the duct. In many houses 20% or more leaks out through joints and seams before it reaches a room, and in an attic or crawl space that air and its heat are lost. Sealing comes before resizing, as the guide to duct leakage explains.