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Friction rate duct sizing method
The friction rate method is the industry standard for residential duct system design. It sizes ducts so that the pressure drop per 100 feet of duct equals the design friction rate. This ensures that every room gets its required CFM when the system operates. Undersized ducts create high velocity, high noise, and insufficient airflow to rooms. Oversized ducts waste money and space.
How to find your design friction rate
The design friction rate = Available Static Pressure (IWG) / Total Effective Length (ft) x 100. Available static pressure is the fan's total external static pressure minus losses at the air handler, coil, filter, and any supply or return grilles. Use the available static pressure calculator to determine this. Total effective length is the longest duct run from air handler to the farthest register, including equivalent lengths for all fittings. Use the equivalent length calculator for fitting losses.
Round vs. rectangular ducts
Round ducts are more efficient than rectangular ducts of the same cross-sectional area because they have a lower surface area-to-volume ratio, which means lower friction loss per foot. A 10-inch round duct carries more air at lower pressure loss than a 10x8 rectangular duct of similar area. When space permits, round or flat oval ducts are the preferred choice. Rectangular ducts are used when ceiling or floor joist depth constrains one dimension. The rectangular to round calculator finds the equivalent round diameter for any rectangular duct.
Frequently Asked Questions
Standard duct design practice recommends friction rates of 0.08-0.15 IWG/100 ft for residential duct systems. The specific design friction rate depends on your available static pressure and total effective length of the longest duct run. A typical residential system with 0.5 IWG available static and 250 ft longest run gives a friction rate of 0.20 IWG/100 ft — slightly high, which means you'd either increase duct sizes or reduce the longest run. Use the available static pressure calculator to determine your specific design friction rate before sizing ducts.
Standard maximum velocities: branch supply ducts in occupied spaces 700 FPM, trunk supply ducts 900 FPM, supply plenum 1200 FPM, return ducts 600 FPM. Exceeding these limits causes audible airflow noise at registers and fittings, which is the most common complaint in residential HVAC systems. This calculator checks velocity automatically and flags any duct that exceeds standard limits. If velocity is too high at the design friction rate, the duct must be upsized regardless of what friction rate sizing would suggest.