Duct sizing is where HVAC system performance is won or lost. The most expensive furnace or air handler in the world cannot deliver comfort if the ductwork is undersized, poorly routed, or leaking. Yet duct design gets less attention than equipment selection in most residential installations, and the consequences show up as hot rooms, cold rooms, noisy registers, and high energy bills.
This guide covers the equal friction method (the standard for residential and light commercial), velocity limits for noise control, the impact of flex duct, fitting losses, and the static pressure budget that ties the whole system together.
The Equal Friction Method
Equal friction is the most widely used duct sizing method for residential and light commercial HVAC. The principle is simple: every duct in the system is sized to produce the same friction loss per unit length (typically 0.08 inches of water column per 100 feet for residential). This ensures that pressure drops are distributed evenly throughout the system, making balancing easier.
The procedure: determine the CFM required for each branch and trunk section. Look up (or calculate) the duct size that carries that CFM at the target friction rate. The ASHRAE duct friction chart or the Darcy-Weisbach equation provides this relationship for round duct. Rectangular equivalents are calculated from the round size using the hydraulic diameter formula.
For example, at 0.08 in. w.g. per 100 ft: 6-inch round carries about 100 CFM, 8-inch carries about 200 CFM, 10-inch carries about 325 CFM, 12-inch carries about 500 CFM. These values are for galvanized sheet metal. Flex duct at the same friction rate requires sizes 1-2 inches larger.
The friction rate you choose depends on the available static pressure. A typical residential furnace delivers 0.50 in. w.g. of external static pressure. After subtracting for the filter (0.10-0.20), coil (0.15-0.25), and register grilles (0.03-0.05 each), the remaining pressure is available for ductwork. Divide this by the total equivalent length of the longest run to get the maximum friction rate per 100 feet.
Velocity Limits and Noise Control
Air velocity in ducts generates noise through turbulence, especially at fittings, transitions, and register outlets. Higher velocity means more noise. The maximum acceptable velocity depends on the application and the occupant tolerance for noise.
Recommended maximum velocities for residential systems: trunk ducts 700-900 fpm, branch ducts 500-700 fpm, and register outlets 400-600 fpm. For commercial offices, reduce these by 100-200 fpm. For recording studios, hospitals, and libraries, reduce by 200-400 fpm.
Velocity is a natural consequence of duct sizing: V = CFM / Area. A 100 CFM branch in a 6-inch round duct (area = 0.196 sq ft) runs at 510 fpm. The same 100 CFM in a 5-inch round duct (0.136 sq ft) runs at 735 fpm, which is marginal for a bedroom branch. Always check velocity after sizing for friction to confirm noise levels are acceptable.
Return air ducts are often the worst noise offenders because they are frequently undersized. The return grille velocity should not exceed 400 fpm. A common residential filter grille sized at 20x25 inches (3.47 sq ft) at 1,200 CFM runs at 346 fpm, which is acceptable. A smaller 16x20 grille (2.22 sq ft) at the same flow runs at 540 fpm, which produces noticeable whistling.
The Reality of Flex Duct
Flex duct is popular because it is fast to install, requires no sheet metal fabrication, and fits around obstacles easily. But flex duct has 3-5 times the friction of sheet metal duct at the same diameter when fully stretched, and much worse when compressed, kinked, or sagging.
The corrugated inner liner creates turbulence that dramatically increases friction. A 6-inch flex duct at 100 CFM has about 0.25 in. w.g. per 100 ft of friction, compared to 0.08 for a 6-inch metal duct. To carry 100 CFM at the same 0.08 friction rate, you need an 8-inch flex duct. This upsizing is not optional; it is physics.
Installation quality matters enormously. Flex duct must be stretched to its full extended length, supported every 4 feet, and kept to runs of 25 feet or less with no more than two bends. A tight 90-degree bend in flex duct can reduce flow by 30-50% compared to a sweep elbow in sheet metal. Sagging flex duct traps air in the belly and further increases friction.
For trunk lines, always use sheet metal. Flex is acceptable for short branch runs (under 15 feet) to individual rooms if properly stretched and supported. For any run over 15 feet or with more than one turn, sheet metal is the right choice despite the higher installation cost.
Fitting Losses and Equivalent Lengths
Fittings (elbows, tees, transitions, takeoffs, boots) create pressure loss beyond what straight duct produces. The most practical way to account for fitting losses is the equivalent length method: each fitting is assigned a length of straight duct that produces the same pressure drop.
Common equivalent lengths for residential galvanized duct: a 90-degree round elbow adds 10-15 feet, a 45-degree elbow adds 5-7 feet, a straight-through tee adds 5-10 feet, a branch tee adds 25-40 feet, and a register boot adds 20-35 feet. These values vary by size and manufacturer, so use the ACCA Manual D tables for precision.
The total equivalent length of a duct run is the physical length plus the sum of all fitting equivalent lengths. A 20-foot branch with two 90-degree elbows and a register boot has a total equivalent length of about 20 + 12 + 12 + 25 = 69 feet. If this is your longest run, you size the friction rate based on 69 feet, not 20 feet.
Reducing fitting losses is one of the cheapest ways to improve duct system performance. Replace sharp 90-degree elbows with long-radius sweep elbows (which have half the equivalent length). Avoid bullhead tees. Use conical transitions instead of abrupt size changes. Every fitting you eliminate or improve frees up static pressure for the rest of the system.
Return Air: The Forgotten Half
Most residential duct problems are on the return side. Builders and installers routinely undersize return ducts and return grilles because they are less visible than supply runs. The result is restricted airflow, high static pressure, reduced equipment capacity, and frozen evaporator coils.
The return duct system must carry the same total CFM as the supply system minus any outdoor ventilation air. If the air handler moves 1,200 CFM, the return system must handle at least 1,200 CFM. A single central return with a filter grille is the most common residential approach, but the grille and duct must be large enough for the flow.
A 20x25 filter grille handles about 1,200 CFM at acceptable velocity. A 14x25 grille is only good for about 800 CFM. If you have a 4-ton system (1,600 CFM) with one 20x25 return, the velocity is too high and the static pressure drop across the grille and filter is excessive. You need either a larger grille or a second return.
In homes with multiple stories or long hallways, dedicated return air paths from bedrooms back to the air handler are important. When bedroom doors are closed, the only return path is through the gap under the door (about 1 inch, which handles maybe 50-75 CFM) and through cracks. If the bedroom supply delivers 120 CFM and the return path only allows 60 CFM, the room pressurizes, the door becomes harder to close, and the system starves.
Duct Leakage: The Silent Performance Killer
The average existing residential duct system leaks 25-40% of its airflow. That means a 1,200 CFM system may only deliver 720-900 CFM to the conditioned space. The rest leaks into the attic, crawlspace, or wall cavities where it does nothing useful but costs the same energy to move.
Duct leakage is measured by a duct blaster test (also called a duct leakage test or DLT). The test pressurizes the duct system to 25 Pascals and measures the airflow needed to maintain that pressure. Results are reported as CFM25 total or CFM25 to outside (leakage to unconditioned spaces only). New construction codes typically require less than 4% leakage to outside.
The most common leak locations are: connections between trunk and branch takeoffs, connections at the air handler cabinet, return air platform joints, and flex duct connections at boots and collars. Mastic sealant (not duct tape, which dries out and falls off within a few years) is the proper sealant for sheet metal joints. UL 181B tape is acceptable for flex duct connections when properly wrapped and secured.
Sealing ducts is one of the highest-ROI improvements in building performance. For a home with 30% duct leakage in an unconditioned attic, sealing to under 5% leakage typically saves 15-25% on heating and cooling costs. The labor and materials cost is a few hundred dollars for a skilled technician. The payback is often under one year.