Friction rate
The pressure loss a duct system is allowed per hundred feet of length, in inches of water column, found by dividing the equipment's available static pressure by the total effective length of the longest run. Every duct in the system is then sized off that one rate.
Also called design friction rate
What is friction rate in duct design?
Friction rate is the pressure loss allowed per hundred feet of duct, in inches of water column, calculated from the equipment's available static pressure and the longest run's total effective length. It sets the diameter of every duct in the system.
Friction rate: how it works
Available static pressure is what remains of the equipment's rated static pressure once the filter, coil, and grilles have each taken their share. Dividing that figure by the total effective length of the longest duct run gives a single pressure loss per hundred feet. A friction chart then converts that rate, at each duct's required airflow, into the diameter for that specific duct.
Where you meet the term
- In a Manual D duct design calculation, as the single number every duct diameter in the system is read against.
- On a friction chart, where the rate is one axis and required airflow is the other.
- In a diagnosis of weak airflow at a distant register, where the friction rate used for design turns out to have been calculated from an undercounted run.
How to check it
- Confirm available static pressure was calculated, not assumed. Available static pressure is the equipment's total external static pressure rating minus filter, coil, and grille losses. A friction rate calculated without that subtraction overstates what the ducts can actually use. Technician
- Confirm the longest run was used, not a typical one. Friction rate has to be conservative enough for the hardest path to hit. Using a shorter, more typical run understates total effective length and produces a friction rate too high for the system's longest branch. Technician
Friction rate: what it is not
- A friction rate is not a fixed industry constant. A figure like 0.10, cited often enough to sound like a rule, is the result of one calculation for one piece of equipment and one duct layout, not a universal default.
- Friction rate is not read off a chart by habit at the design stage. It is calculated first, from available static pressure and total effective length, and only then used to read the chart.
Friction rate questions
Does friction rate change if the filter is upgraded later?
Yes. A higher-restriction filter reduces available static pressure, which lowers the friction rate the ductwork was actually designed to and can reduce airflow across the whole system, not just at the filter.
Is a lower friction rate always better?
A lower rate means larger, quieter ducts as a rule, but it also means more material and more installed space. The rate that matters is the one the equipment's available static pressure and the run's total effective length actually support.
Can two duct systems on the same equipment have different friction rates?
They can if they are designed to different total effective lengths, but doing so is unusual. A single friction rate applied uniformly is what lets every duct in one system be sized from a single chart lookup.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Manual D residential duct systemsACCA description of Manual D as the residential duct design procedure that derives a friction rate from the equipment available static pressure and the total effective length of the longest run, then sizes every duct from that single rate, with fittings converted to equivalent lengths rather than counted as physical distance.