6' ducting on island hoods converting to larger?

frank04090March 13, 2012

I'm looking at a few different hoods for a peninsula. Trying to stay under $1,500.

There are some we like from Faber, Zepher, and XO but they are all 6" ducts...

My understanding was if the spec says 6", you are stuck with 6". You can run an 8" adapter pipe to the roof after it leaves the hood but your 6" on the hood is a bottleneck. Then, while looking at the faber website I saw this in the FAQ:

"Faber hoods should be ducted using ridged smooth metal ductwork. See the installation instructions for specific guidelines, but in general most hoods use 6 inch round ducting. The basic rule of thumb for ducting, is keep it short and straight! The shorter the duct run the better, and the less elbows in the duct run the better. Generally 35 feet is the maximum duct run, adding 5 feet for every elbow used. Never use two elbows back to back in a duct run, and seal all joints with duct tape. Also, reducing the size of the duct is never recommended, you can however over size the duct to have less pressure. For example if the hood uses 6" round ducting, you can convert to an 8" round duct. "

My question is this: If you "convert", is it a separate piece that you buy to get rid of any 6" duct or is it just an adapter somewhere above the ceiling that connects your 6" hood to an 8" system?

We are looking at running a 600+CFM system. We rarely run more than one burner at a time.

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kaseki

You would have to contact the manufacturer to determine whether any particular unit is adaptable to an 8-inch duct without the significant transition pressure loss of abuting 8-inch duct to a 6-inch hole. Otherwise, a proper tapered transition piece should be used to reduce the potential transition loss.

kas

    Bookmark   March 13, 2012 at 8:29PM
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davidro1

I would upsize to 7". The 6" exit on the hood is NOT a bottleneck. Search Venturi effect to know more about why a single pinch point is not a bottleneck for fluid flow.

Transitioning to 7" is easy. You buy a transition piece for less than $10.

    Bookmark   March 17, 2012 at 7:54AM
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kaseki

As far as I have ever observed, venturis (like those used in 80 years worth of carburetor designs, 60 years of turbojet designs, and pictures on Wikipedia) have smoothly varying ramps on each side of the constriction.

If the assertion intended that there is no significant increase in pressure loss for an existing 6-inch hole mated to a transition piece, I would agree. If the assertion intended was that there would be no increase in pressure loss by abutting a 6-inch hole to an 8-inch duct without the transition piece, then I would disagree. Better would be a larger hole that matched a larger duct. Even better would be a NACA throat at the hood mating to a larger duct.

kas

    Bookmark   March 17, 2012 at 5:23PM
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frank04090

Ok so it's not a bottleneck then. Excellent, thanks for the info.

    Bookmark   March 17, 2012 at 11:41PM
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