By Martin Harris.

Dust Collection Ductwork Setup Guide

Ductwork is where most small shop dust collection systems quietly lose their performance. You can own a genuinely strong collector and still end up with weak airflow at your tools if the duct between them is undersized, too long, or full of unnecessary bends. This guide covers the actual numbers behind duct sizing, based on published manufacturer data, not guesswork.

Main versus branch sizing

A dust collection system typically has one main duct run from the collector, then branches off to individual tools. The main line should be sized to carry your system's full target CFM without excessive velocity loss, while branches can step down toward each tool, though not necessarily all the way to the tool's stock port size.

Woodcraft's central system design guide describes a representative layout starting with a 6 inch main, 5 inch branches to larger machines like a table saw or jointer, and 4 inch branches for smaller tools. The exact numbers depend on your collector's capability and your tool list, but the general pattern, stepping down gradually rather than running one uniform size everywhere, holds up as a reasonable default for most small shops.

Static pressure: the number that actually matters

CFM tells you volume. Static pressure tells you resistance. A collector's rated CFM is measured under close to ideal conditions; your actual duct network subtracts from that number at every foot, every fitting, and every transition. Oneida publishes a component-loss table, measured at roughly 4,000 feet per minute, that makes this concrete:

Values in inches of water column. Source: Oneida Air Systems static pressure component data, measured at approximately 4,000 FPM and sea level.

A few things jump out looking at this table. First, one foot of flex hose costs nearly as much static pressure as five feet of rigid duct at the same diameter. That is why every guide, from Pentz to Oneida to Woodcraft, says the same thing: keep flex short. Second, a single 90 degree elbow costs more than five feet of straight rigid duct, sometimes by a wide margin. Routing your duct with 45 degree bends or wye fittings instead of hard 90s, where your shop layout allows it, is a real, measurable improvement.

If you are at elevation, note that Oneida's published altitude correction factors rise from about 1.03 at 1,000 feet to about 1.64 at 10,000 feet. Static pressure losses increase as air density drops, so shops at meaningful elevation should budget for somewhat higher resistance than sea level numbers suggest.

Transport velocity: keeping dust moving

Static pressure is about resistance. Transport velocity is about whether your duct actually keeps chips and dust suspended and moving, rather than settling and building up inside the pipe. OSHA's woodworking guidance cites roughly 2,500 to 4,000 feet per minute as a duct velocity range for moving sawdust, chips, and shavings. Spiral Manufacturing's industrial design guide is a bit more specific for woodworking: around 4,500 FPM in branch lines and 4,000 FPM in mains. Bill Pentz's independent, more conservative guidance lands around 3,000 FPM horizontal and 4,000 FPM vertical, with higher velocities recommended for larger chip material.

These numbers do not always agree exactly, and that is fine. What matters is understanding the relationship: CFM divided by duct cross sectional area gives you velocity. Too large a duct at too low a CFM drops velocity below the point where chips stay suspended, and material starts settling inside the pipe. Too small a duct at high CFM increases velocity but also increases static pressure loss dramatically, since resistance rises fast as diameter shrinks. Sizing duct correctly means balancing both, not maximizing one at the expense of the other.

Practical layout guidance

• Minimize elbow count over minimizing distance. A slightly longer run with fewer, gentler bends often outperforms a short run packed with tight 90 degree turns, based on the loss numbers above.
• Use wyes instead of tees where branches join the main. A wye fitting introduces the branch air at an angle closer to the main flow direction, generally producing less turbulence and loss than a sharp tee.
• Keep flex hose to the last few feet at each tool. Flex is for the final connection and flexibility at the machine, not for covering distance across the shop.
• Support duct runs properly. Sagging horizontal runs, especially in flex hose, can create low points where dust and moisture collect, contributing to clogs over time independent of the sizing math.

Recommended gear

For planning your full layout, start with my dust collection system design guide and my CFM calculator to run your specific numbers. For pipe material and the metal versus PVC question specifically, see my dust collection pipe comparison.

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