Is it OK if a vent is crushed?

10 years ago

My contractor's sub put in a 6" vent for my island hood because that was convenient for him (more work to put in a larger one because of space constraints). That 6" hood is less than ideal to begin with because I'll have a powerful hood.

Possibly making matters worse, there is a point where the vent is compressed a little so it's 5" high (but obviously a little bit wider than 6" as a result). The reason is the vent is pinched between a water line and gas line at this point.

My contractor says this is OK because the volume doesn't change. Of course, this sounds logical, but things don't always follow logic. Should this issue be fixed or is this OK?

Featured Answer

bry911

10 years ago

last modified: 10 years ago

My contractor says this is OK because the volume doesn't change.

That is not correct. The area of a 6" circle is 28.27 square inches and therefore the volume is 28.27 x the height. The area of a 6" circle stretched into a perfect 5" oblong would be 27.48 square inches.

Neither of these things say that his conclusion is wrong. It may still be fine, but his reason is not correct. Really though the loss is not that much. However, while distorting a round pipe may have no appreciable effect, seriously creasing the pipe probably will.

Just remember that a circle is the most efficient shape (area inside/perimeter distance) and the farther from circle the less efficient (an octagon is more efficient than a square and a square is more efficient than a triangle, etc.).

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Comments (8)

cpartist
10 years ago
I would ask on the appliance forum.
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just_janni
10 years ago
If you should have had an 8" to begin with (and I guess that you should have based on your comment - or gasp - maybe even a 10"?) then squashing a 6" IS material - as bry911 mentioned. Unfortunately = rerouting it and adding length and bends might negate the better flow if you could maintain the full 6". It's hard to add HVAC after all this other stuff and that's why I recommend putting it in early - or at least mocking it up.
daveho
10 years ago
I'd also be concerned about the vent possibly touching the water & gas line. The possibility for additional noise or rub through would bother me.
User
10 years ago
last modified: 10 years ago
The minimum round metal duct diameter for a range hood is 6". Higher performance (greater fan size) hoods may require an 8" or 10" round duct.

Compressing the duct an inch shouldn't be a problem but if the duct is undersized the fan will be working very hard at the high fan setting and work fine at lower settings. Frankly, I think high performance range hoods are misunderstood and overrated. When you crank one up over 400 CFM it won't work properly unless you open a door or window and doing that would probably resolve the problem by itself with less noise.

So, what are the fan speeds and the manufacturer's recommended duct size?

In certain jurisdictions a fan speed greater than 400 CFM will require an automatic makeup air system in order to prevent a negative air pressure condition in the house.
uscpsycho
Original Author
10 years ago
last modified: 10 years ago
I haven't settled on the hood yet but it will most certainly be a model over 400 cfm, probably in the 600 range and possibly up to 900 (because I love the way it looks). I don't think I'll have the problem you describe since my kitchen is large and open. And it's an old house so not all that air-tight. I don't have MUA but it's not required by code. My contractor looked at my funny when I brought it up and insists I don't need it.

Here's a slightly off topic question that I should probably ask in a new thread. And it's probably a stupid question because I'm pretty sure I know the answer. But if I replace this 6" vent with an 8" vent, I won't have a problem if I use a hood that calls for a 6" vent. The 8" vent can be connected to that hood. Right?
uscpsycho
Original Author
10 years ago
Anyone else think there could be a noise problem from the vent, gas and water lines being in close proximity? If so, I assume it won't be an issue if we make sure none of them are touching each other. Any noise coming from this location would certainly bother me; I brought it up to my contractor and he says it's not going to be an issue.
User
10 years ago
Why not use a rectangular duct that would fit between the pipes without touching them. Exhaust hoods have provisions for round and rectangular ducts or you could use a transition fitting on each side of the obstruction. I suspect the GC is doing this himself and prefers round ducts.