Dust Collection Ducting: 4-Inch vs 6-Inch Mains Explained
Dust Collection Ducting: 4-Inch vs 6-Inch Mains Explained - A step into the workshop.
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There is a moment in every garage-shop build where you stop dragging the flex hose from tool to tool and start thinking about pipe. Your wrist hurts from coiling it. The collector is loud in the corner. And you've started to notice that the fine dust doesn't end up where the dust shoe is pointed. So you sit down with a catalog and a notepad, and within twenty minutes you're staring at two numbers that will govern the rest of the decision: four inches and six.
I've come to believe the useful way to choose between them is to stop treating the main as a shopping decision and start treating it as a fluids problem. The pipe has one job: keep sawdust moving at a speed where it stays in the air, all the way to the collector, on the worst day, with the blast gate at the end of the run open and every other gate closed. Everything else is budget.
The Transport Velocity Constraint Comes First
Dust that falls out of suspension in a horizontal pipe settles and stays there. That's the failure mode that matters in a hobby shop, because a slumped pile of fines in a run of main is a restriction you have to clear.
The design number I've seen most often quoted is transport velocity: the minimum air speed needed to keep material in motion. One calculator site, WoodFormulas, states this as 4,000 feet per minute, with sawdust "drops out of suspension below 3,500 FPM." I want to be clear that this is one source's stated rule of thumb, carrying an affiliate disclosure, citing Darcy-Weisbach and ACGIH Industrial Ventilation as its basis — not an industry standard I can independently confirm. Treat it as a working target, not a code citation. But the logic behind it holds up: you cannot evaluate a duct diameter without also knowing what flow rate you're trying to push through it, because velocity is just flow divided by area.
Do the arithmetic with me. A six-inch round duct has a cross-sectional area of about 0.196 square feet. At 4,000 FPM, that's roughly 785 CFM. WoodFormulas states that number explicitly, and it matches the geometry. Those two figures — the transport velocity and the flow a six-inch main needs to reach it — are the entire argument.
Why the Six-Inch Main Is Not Automatically the Upgrade
Here is the part that surprises people who come from general construction, where bigger duct is always better because it moves more air. Dust collection is not a supply-air problem. The fan has a fixed pressure budget, and the pipe spends it.
WoodFormulas claims that air friction in a four-inch duct is "more than three times higher" than in six-inch at the same volume. That's the standard argument for going large, and I don't doubt the direction of it. But that comparison only holds at the same volume.
I want to be honest with you about the evidence level here. The research for this piece could not verify a primary-source static-pressure comparison between four- and six-inch mains, and could not obtain fan curve data showing where any given collector stalls. What I can show you is the geometry, which is not a claim from a source — it's arithmetic. A six-inch main requires roughly 785 CFM to hit the transport velocity WoodFormulas describes. Whether your collector can supply that flow at the static-pressure load of your run is the question you have to answer before you buy pipe, and I could not verify the answer for any specific machine.
That is the honest version of the "six-inch main starves a small collector" idea: not that six-inch pipe is bad, but that it needs more air than some collectors can supply, and it needs it at the far end of the run. Whether yours is one of them is a matter of the fan curve, not of folklore.
The Flex Hose Is Where the Numbers Get Ugly
WoodFormulas makes two claims about flexible hose that change how a garage system should be laid out. First, that each foot of flex is approximately equivalent to three feet of rigid pipe in static-pressure loss. Second, that flex drops should be kept to four to six feet maximum. Both are rules of thumb from a single calculator site, and I'd flag them as directional rather than precise.
But they point at a layout principle I've come to accept: hard-pipe the fixed runs, and use flex only for the last few feet to the machine, where you actually need articulation. The common failure in a small shop is the opposite — a rigid main with three-foot flex drops to every tool, each one a hidden multiplier on friction. Add a blast gate at each drop and you have a run that looks tidy on paper and performs like a straw.
The same source claims advertised "free air" CFM drops 40% to 60% in a real installed system. I'd treat that range as a warning, not a measurement. It is the reason to be conservative when you're matching a collector to a duct, and the reason a machine rated generously in a catalog can feel weak at the tool.
What a Oneida-Priced Run Actually Looks Like
The assignment asked for a priced bill of materials for a roughly 20-foot hard-piped install. I could not assemble one from verified sources: blast-gate and flex-hose prices were never successfully fetched, and I will not invent them. What I can give you is the verified Oneida Air Systems list pricing for the components that dominate a run like this, which tells you where the money actually goes.
| Component | Oneida list price (at time of writing) |
|---|---|
| Snap-Lock Piping | as low as $12.95 |
| Spiral Piping | as low as $39.95 |
| 45° Wye Joint | as low as $49.95 |
| Long Radius Adjustable Elbow | as low as $19.95 |
| Pants Wye Joint | as low as $199.95 |
| Aluminum Foil Tape, 2" × 165' | $19.95 |
| Sheet Metal Screws, #8 × 1/2", 100 pk | $3.95 |
| Industrial Grade Silicone Sealant | $9.95 |
| 4" Ductwork Wall Hanger Kit | $39.95 |
Two things stand out. First, the fittings are serious line items. A 45-degree wye at $49.95 and a pants wye at $199.95 are not rounding errors next to pipe that starts at $12.95. Second, Oneida sells ductwork in 54 listed items across standard, quick-clamp, misc, and package groups, and offers ductwork starter packages. If you are pricing a garage install, look hard at the packages before you build the run a la carte.
Oneida also offers a $400 Professional Design Service fee, credited back to you when you purchase your ductwork from them. For a client commission where the shop is going to be used daily, I'd treat that as cheap insurance on a system that's expensive to remediate after the fact. For a single-car garage with two machines, price the packages yourself first.
The Layout Rules I'd Actually Build To
Synthesizing the above into layout choices, with the caveat that two of the three rules come from a single unverified calculator source:
- Size the main to the machine, not to the future. Get your collector's fan curve and size the main so velocity stays up at the flow the machine can actually deliver at your run's static pressure. A six-inch main needs roughly 785 CFM to reach 4,000 FPM; if your collector can't supply that, four-inch is the defensible choice.
- Keep runs short and straight. Every fitting adds static pressure the fan has to pay for. WoodFormulas recommends 45-degree wyes and long-radius elbows at 1.5 to 2.0 times diameter, and Oneida's long-radius adjustable elbows at $19.95 are the fitting to put in that slot.
- One machine at a time. A hobby system with blast gates is built to move air through one open gate at a time. Design for that case, and don't expect two open gates to perform.
- Use flex only at the tool. Keep the drops to four to six feet per WoodFormulas, and hard-pipe the rest.
- Seal every joint. The $19.95 foil tape and $9.95 silicone in the Oneida catalog are not optional line items. Leaks in a small system are the same as shortening the pipe.
The Judgement
I've come to believe the four-inch-versus-six-inch question is misframed when it's asked as "which is better." Better than what, at what flow? The honest framing is: compute the flow your collector can actually deliver at the static-pressure load of your proposed run, then size the main to keep velocity up at that flow. Whether that arithmetic favors four inches or six depends entirely on the machine, and I could not verify the answer for any specific collector.
Before you buy any pipe, get your collector's fan curve in front of you and do the division. If you can't get a fan curve, call Oneida's design service and ask what main size they recommend for your specific machine — and weigh the $400 against the cost of re-piping a garage after the fact. The design is the cheap part. The fittings are not.