Difference between a simple 5” duct versus a transition hood/boot in bottom of miter

[Bruce Seidner]

I’m building a miter saw hood that will be ducted at the bottom with a 5” pipe into a 6x5x3 wye with a flexible 3” duct to the blade guard.

I can run the 5” duct straight into the bottom of the box and have a 5” round hole at the bottom. I can fix a 4”x10” HVAC boot to the bottom that transitions to a 5” duct leaving a 4”x10” rectangle at the bottom.

The labor is a wash. The boot costs $15 which, in the scheme of things, is also a wash.

Objectively, is one option more efficient than the other? What difference in extraction would there be, if any?

[Paul F Franklin]

I don't think it makes much difference. The area of the 4x10 is quite a bit more than 5 round, but since it's going to a 5" duct that's going to be the limiting factor.

The larger opening might reduce the little piles of sawdust that tend to accumulate in the corners; and if you can slope the bottom of the box toward the boot, that will help that too. But it's not like it's a big deal to reach in a brush the little piles into the duct once in a while.

[Jack Frederick]

The net effective flow in a rectangle is really an oval but I am with Paul. You have twice the cross sectional area. You might consider using a 4x10 “end boot”. Off that you go to a 45 that drops into the 6” mains Wye. I think the flow would be better. With all the nooks and crannies on those saws it is a good idea to have compressed air there to lift all the crap that falls out.

[Bill Dufour]

I would make that slot no more then 2" wide to keep velocity up. On my TS I used a piece of 6" pvc pipe and cut a 2" wide slot for the blade. It is capped at one end.
If your dc hose has to exit downward I would buy a tee and two end caps. Glue on two short nipples and the endcaps so they are inline. Then cut a slit opposite where the extra pipe will attach. Make it about 2" wide and 10 inches long
Bil lD

[Jim Becker]

Since you are already pulling off a branch for the saw hood, reducing for a 4" boot shouldn't present an issue, IMHO.

[Bruce Seidner]

These are all very helpful observations and ideas. I think the issue that I wondered about is "trouble" they go in the transition from the round duct to the open slot of the boot. It is a pretty elegant sheet metal origami and I am thinking they have a sound engineering concern to minimize turbulence in so doing. I will experiment with the size of the slot and do want to maintain the velocity. With a 3" blade guard duct and a 5" hood duct feeding the same 6" duct all roads lead to Rome. It just might do a little better job splitting the load like this. I am working on a little hood to replace the very inadequate blade guard hood on my DeWalt 12" MS. The more that gets caught right out of the chute the less there is to disburse in the hood.