# Boots and hoods versus straight duct efficiency



## Perplexed1ne (Apr 23, 2020)

In terms of efficiency and loss of CFM, what is the difference between using a HVAC tapered air handing boot at the bottom of a miter saw hood versus just cutting a hole for the 6" duct?

The cost is about ten bucks and difficulty is essentially the same. You can install an HVAC boot on the bottom of a miter saw hood which is a 4"x10" slot and nifty sheet metal that transitions to a 6" duct. You can cut a 6" circle in the bottom of the miter saw hood and glue in the duct.

Anyone have any experience with this?


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## therealSteveN (Oct 29, 2016)

I think most people are still looking for the "best" way to control miter saw dust. If you can get most, you are winning. Anyone who collects ALL gets to write the book.


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## rwe2156 (May 7, 2014)

What Steve said^ is very true. The objective is enough collection you don't need a mask.

I used a wall mount 1 HP blower modified to sit on the floor and exhaust outside, 4×10 scoop with a grille to prevent chips from entering blower.

Combined with the sliding doors for miter cuts, I'm very happy with the collection,



















,


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## Perplexed1ne (Apr 23, 2020)

That's a nice set up.

I was wondering if there was some increased efficiency, not just at the bottom of the hood, but in terms of capture and flow across the whole hood and above the 4'x10' slot or 5" hole. They spend a lot of time engineering transitions from one size duct to another and from round to rectangular duct. I understand these efforts in terms of reducing CFM killing turbulence. Maybe the long slot would be more effective as a transition into the 5" round duct if it was a proper engineered transition like a floor register boot. There must be some reason they go to that trouble instead of just using round pipe into the rooms of a house. Likely overthinking this, but I find it interesting and it's not expensive to implement.


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## clagwell (Dec 20, 2018)

HVAC boots are designed for air flowing OUT of the boot. For dust collection the air flows IN. Flow patterns and losses are completely different between the two.

The most efficient entrance is a Bell Mouth. That is, a round opening with a rounded edge:









Pressure loss at the entrance will vary with the inverse fourth power of the diameter, so bigger is better.


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## Perplexed1ne (Apr 23, 2020)

This makes perfect sense and makes my work so much easier. Thank you.


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