Vent-a-hood- reducing the duct size?

16 years ago

We have been planning on getting a Vent-a-hood for our kitchen remodel. However, we're now looking at the specs, and it needs an 8" duct. Does that 8" duct have to be the whole way from the hood itself to its termination point outside? Or can you reduce the diameter of the duct after its initial protrusion from the hood?

I ask because we do not have any bulkheads or anything to hide a duct in, especially an 8" duct, and because of the galley configuration of our kitchen, we can't run it between the joists, as the duct needs to run perpendicular to the joists.

If anyone has any thoughts about reducing the diameter of the duct on the hood, I'd appreciate it.

Thanks!

K

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Comments (9)

mminkous
16 years ago
It will have to be 8" all the way, or whichever way as recommended by VAH. Best thing is to call them to confirm.
weissman
16 years ago
If you reduce the duct size you'll reduce the effectiveness and increase the noise of the hood. There are a few hoods that are spec'd for 6" ducts.
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That is irresponsible of your builder and is a warning sign about their knowledge, quality and integrity. Reduction from 8" to 6" alone is a problem. It will reduce air flow and so the effectiveness of your hood and it will increase hood noise as well as the amount of electricity used. So increased cost, more noise, less odor, gas combustion by-products and other effluent removal. Flex duct creates two problems; It's a major fire hazard and it increases static pressure (reduces air flow). From a static pressure standpoint 6" flex is roughly the same as 4" rigid in most installations. At a MINIMUM you should have 8" rigid for your entire run if that is what your hood calls for (Many US hoods undersize this). If you have many bends then you should increase that to 10" to keep static pressure down.
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No disrespect, but I disagree with Davidro1. And there have been many threads on duct size on this forum, but they may have fallen off by now. IMHO, it is crucial to have a duct that is 8" or 10", and not 6". I speak from experience. I have a 6" duct, and it is the ONE disappointment in my kitchen reno. The ONE thing I wish I could change. Unlike you though, I did not have the luxury to install ductwork for my exhaust; I was locked into what was already there, which is 6" size. If you are lucky enough to be able to choose, get the 8" (many on this forum have 10", but 8" will be fine). Don't let your contractor push you around. Really, this is important. Ask yourself--what difference does it make to him if I get an 8" duct? Well, it'll probably be a bit more work for him to squeeze in an 8" duct rather than an easy-to-slide-in 6", so that may be why he's telling you 6" will be fine. It's YOUR kitchen that you'll have to cook in for years, long after he's gone and forgotten. Tell him you want the 8" duct installed, no debate. You have already hit on one reason not to get a 6"--it will severely limit your choices in hoods. Meanwhile, if you have an 8" duct, your choices will be wide and varied at all different price points, designs, brand names. A second reason for a larger duct is efficiency. A 6" duct simply won't be large enough to allow for cfms above 300-450. 300-450 may be adequate, but on those days when you are frying hamburger or fish, you'll wish for more. Believe me, I know. As to overkill--I would rather have high cfms that I may only use once in a while, with the flexibility of lower settings for the rest of the time, than to be permanently hampered by low cfms due to a 6" duct size. Per the Appliances Forum FAQ on Vent Hoods: "The rule-of-thumb is 10CFM per 1000BTU of burner capacity. Thus, for a 60K-BTU cooktop (4 x 15K), you should have a 600CFM hood. Since you rarely, if ever, have all burners on full at once, you have excess capacity that can be used when you really need it. Most hoods have variable-speed controls, which allow you to choose the airflow---and noise level---appropriate to the task." Another reason for a larger duct is noise level. When you are pushing air through a narrower space, it will be more noisy than pushing the same air through a wider space. Of course, the length of your duct run and the number of bends will also affect noise level, but 6" will guarantee a higher noise level than 8". Although duct size has been discussed a lot on this forum, I have never once seen a thread saying "AW SHOOT, I WISH I'd installed a smaller duct!" On the contrary, we see the opposite sentiment, "If only I had a larger duct..."
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The answer to the question is use the larger duct for as much length as possible. From the description, it seems that it could be 6-inches over most of the length, raising the question of why the last bit couldn't be cut larger. Note that duct transitions should be gradual and not abrupt. Your 400 CFM rated blower can only do some lesser flow rate when subjected to pressure loss, some of which is in the hood itself, some of which is in the make-up air path, and the rest in the ducting. Suppose you could actually achieve 200 CFM. Then 4-inch duct would have to be flowing 2300 ft/min, a higher value than desirable and likely to have more pressure loss than the 200 CFM assumption might have assumed. So maybe 100 CFM. You will be in the average condition of 1950s hoods, so all you need is a larger duct to move into th '90s. :)
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The 4-inch duct was too small for the original plan. Probably an 8-inch would have been needed and then the higher flow rate requirement could be made to put up with a bit more pressure loss than is desirable . Air velocity in the hood ducting should be in the 1000 to 2000 ft/min range. Even if you only put a short length of 4-inch duct in a 10-inch duct path there would be very high pressure loss. (On-line calculators exist for these types of analyses.) And if you used a screaming mimi aerospace blower to get the flow rate at the pressure loss, the duct air velocity would be absurdly high and would try to compete with the blower for noise champion. Venting down and out may not be a violation, so long as your AHJ is OK with the details including slopes, sealing, and access, but such paths tend to be restrictive for various secondary reasons. And it seems you are limited to 4-inches in that direction, if I understood your message. I have no real picture of your kitchen, but if it were my kitchen I would use a 10-inch duct from the hood to the nearest exterior wall that didn't dump onto a deck or an openable window. I'd either enclose the duct with a soffit, or make it decorative. Alternatively the duct would go to a point on the ceiling where an upstairs chase could be added to get the path to the attic where it can deviate as necessary to get to a good position for a roof blower. For reference, I have seen images of wood-everywhere type structures where a hood duct went skyward across the room to the desired point on the roof. This appeared to be at least 20 ft high. Such schemes require stainless steel ducting to look decent. This wouldn't do if the ambience were not rustic. Can you extend the wall studs on the existing wall by 6 inches (cutting out 6 inches of space next to the cooktop) so there is space to have enough duct area?
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cpovey
16 years ago
A 8" diameter vent provides approximately 25" square inches of duct space. Can you find space for a rectangular duct that provides the same square inch space, say like 4x6?

If you can fit in a rectangular duct, keep the transitions (round to rectngular, one size to another, etc.) to the absolute minimum. Every transition is a source of noise.

Either way, reducing the duct size dramatically decreases effectiveness and increases noise. If you can only fit a 6" duct, you might as well put in a less-powerful hood- a larger fan simply does not increase the amount of air flowing through a pipe.
solarpowered
16 years ago
Point of arithmetic, an 8" duct has approximately 50 square inches of cross-section.
jamesk
16 years ago
An 8" round duct is required from beginning to end (including the termination on the roof or outside wall) for a 600CFM Vent-A-Hood. Alternatively, you can transition from round to rectangular ducting, as long as the rectangular ducting is equivalent in size. An 8" round duct encloses approximately 50 square inches.

You don't mention the type of cooktop or range that you will be venting. If it isn't a pro-style gas range, you very likely could go with a 300CFM model which requires a 6" round duct (approx. 28 sq. in.).

I have a 600CFM Vent-A-Hood over a 36" Gaggenau gas cooktop. About 95% of the time it runs on the lowest SensaSource setting with only a single blower going. It vents very effectively (and quietly) at that speed. Unless I'm cooking something over very high heat that generates a lot of smoke, I almost never turn on the second blower.

Your contractor should be able to find a way to install the appropriate sized ducting. If not, call an HVAC/sheetmetal contractor. Installing a new, properly-sized duct through the attic, through joist spaces, or through a side wall shouldn't pose an insurmountable problem.
heimert
16 years ago
not knowing the layout of the kitchen, it's tough to say, but what about a soffit above the cabinets? If you're having trouble running 8" why is 6" any easier? The idea of going rectangular should be able to achieve the same thing. Do a transition at the elbow at the ceiling to 6x9 rectangular and you'll be fine.
cpovey
16 years ago
Oops 50 square inches. My bad.
Tom Pultz
16 years ago
I get the impression reading the information on the VAH website that their hoods are underpowered and will not work too well against excessive back pressure. For instance, the outside wall hood sold by VAH does not include a spring loaded damper door and they say you should not use one. I'd stick with 8" all the way if you can.
fausonk
Original Author
16 years ago
Thanks for everyone's input. Honestly, I hadn't thought about where the vent was going to go, so a 6" or 8" or whatever isn't the problem-- having anything at all is the problem! Smaller would be less intrusive, though. So, looks like 8" is required the whole way. We need to figure out how to get that sucker out the wall (or up through our second story and out the roof) without it looking horrible. We've managed to arrange everything in our house to not have soffits/bulkheads but looks like the vent ducting is going to kill that!

Thanks for all your input and suggestions!
K

Vent-a-hood- reducing the duct size?

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Vent-a-hood- reducing the duct size?

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