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Old 01-08-2016, 03:31 PM   #1
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Venting a range hood down thru the besement


I know this topic has been raised before, but each person's situation is different, so I thought I would sketch out mine and ask the DIY community for advice and insights.

Our typical Whirlpool gas range sits in our condo's kitchen against a 4" interior wall. There's a microwave above the stove with a recirculating fan system that speeds up the time it takes to stink up the place every time I do a stir fry or sautee some onions. I would like to replace the microwave with an under-cabinet range hood, but there is no reasonable way to run venting to the roof (there's a small office above the kitchen) or an exterior wall, given the direction of the studs in the ceiling.

I removed the microwave the other day and could see an installed piece of 3.25x10" ducting in the wall behind it. The duct's opening faced the kitchen, then did an abrupt 90 degree turn downward, continuing down between the wall studs maybe 7 or 8 feet to end abruptly high on the basement wall, right where it meets the ceiling.

I am thinking about buying a range hood with a 3.25x10" opening in the back of the hood that I can connect to the existing vent, which I can move around a bit to line up with the hood. I would then use an adapter to connect to the duct in the basement, angling downward about 20 degrees, while transitioning to an 8-inch round duct. That duct would continue about 10 feet to the sill of the building, where I'd exhaust it.

To power the system, I would avoid buying a fan blower for the hood, since that would just blow the air directly into the duct's initial 90-degree bend, creating a lot of back pressure. Instead, I'm thinking of installing an inline 600 cfm blower, which would hang from the basement ceiling somewhere along the 10-foot stretch of 8" round duct. The fan comes in a housing so it's easy to connect the round ducting to it.

Thus, the inline fan would suck air down the wall duct, rather than try to push the air down.

Do you think this approach would work? I'm wondering if applying 600 cfm suction to a 3.25x10" vent would create a lot of noise and/or provide good suction within the hood. I should add that the hood I'm looking at has an adjustable knob that lets you infinitely adjust the fan speed, so I could reduce the suction if the noise were too extreme.

Your help is always appreciated!
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Old 01-10-2016, 02:44 PM   #2
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So if I understand what you are saying, the piece of duct you found in the wall isn't connected to anything down below? Not sure if the current duct would handle 600 cfm. Double check on that and I don't see why it wouldn't work. The other thing to consider though is make up air. Depending on how tight the structure is, once you start getting above 2-300 cfm, that's a lot of air leaving the house that needs to be made up.
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Old 01-10-2016, 03:11 PM   #3
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Exactly right, Mike- the ducting in our condo wall dates back before our move here a year ago. It ends abruptly, but would be easy to tie into.

I'm looking at in inline blower rated at 600 CFM, made by BEST, a division of Broan. The blower sits in a metal box, which comes with adapters for 10" round ducting. So I would have to transition the 3.25x10" duct to a 10" round to enter the fan box.

The trouble is that, though the blower also exits with 10" ducting, I can't fit a 10" or really even a 7" pipe through my sill. Well, in a pinch I could probably squeeze a 7" piece, but I'm thinking about transitioning down to a 3.25x10" exit duct that would connect to a standard wall cap. I've seen hoods on the market rated at 600 CFM that use these 3.25x10 ducts, so I'm thinking this is doable. Plus, since the initial vertical drop to the basement uses this size, would it be a bad decision to use this kind of duct for the exhaust end?

The thing that gives me pause is the manufacturer's spec ing big 10" ducting for the fan blower.
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Old 01-10-2016, 09:31 PM   #4
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i would : get a hood, properly install it. then see how it works. go from there.
you may be over thinking this.
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Old 01-11-2016, 08:06 AM   #5
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Originally Posted by Fix'n it View Post
i would : get a hood, properly install it. then see how it works. go from there.
you may be over thinking this.
Well, the devil's in the details.


The reason I started this topic was to get advice about what the proper way was to install a hood and-- more importantly-- the fan blower and ducting.
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Old 01-11-2016, 08:52 AM   #6
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Originally Posted by Amazer98 View Post
Our typical Whirlpool gas range sits in our condo's kitchen against a 4" interior wall. There's a microwave above the stove with a recirculating fan system that speeds up the time it takes to stink up the place every time I do a stir fry or sautee some onions.
I just thought I'd mention, at our getaway cabin the range is on an exterior wall. There is a 12" through-the-wall exhaust fan directly above the stovetop and under the hood. Even that doesn't stop the smell from stir-fry, sauteed garlic, etc, from quickly filling up the entire joint.
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Old 01-12-2016, 10:15 AM   #7
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Did you try calling the blower manufacturer and talk to their tech department about your install? They may be able to tell you one way or the other. I was looking at different specs for duct sizes and that seems pretty straight forward. But then I also went to Broan's site and looked at some of their specs for different rated cfm units. Doesn't seem to jive.
http://www.waptac.org/data/files/web...ck%20chart.pdf

You could try what you mentioned above, if for some reason the necking down to 3.5 X 10 at the end of the run causes problems, maybe you could fit a larger size pipe through the sill. Maybe a larger rectangular size instead of larger round tube.
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Old 01-12-2016, 11:38 AM   #8
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Quote:
Originally Posted by firehawkmph View Post
Did you try calling the blower manufacturer and talk to their tech department about your install? They may be able to tell you one way or the other. I was looking at different specs for duct sizes and that seems pretty straight forward. But then I also went to Broan's site and looked at some of their specs for different rated cfm units. Doesn't seem to jive.
http://www.waptac.org/data/files/web...ck%20chart.pdf

You could try what you mentioned above, if for some reason the necking down to 3.5 X 10 at the end of the run causes problems, maybe you could fit a larger size pipe through the sill. Maybe a larger rectangular size instead of larger round tube.
Mike Hawkins
Yes, I called Broan/BEST (same company) tech support at least 3 times. The first two times I was told that my installation was possible, but not recommended. Today my sales rep forwarded an email from the Broan regional manager that pointed out that the rear 3.25x10" opening was only to be used with the optional internal blower (i.e., the rectangular opening would not accommodate the required electrical connections, plus there would be a large rectangular opening in the top of the hood that would have to be sealed somehow).

If I used the 10" round duct out the top of the hood, I would still have to figure out a way to transition with very limited room to the wall duct. Maybe turn those small 12" cabinets above the stove into a plenum box?

So.. it might be possible to use the hood, but it seems like it's getting complicated. I'm starting to consider the Vent-a-hood ARS system, which is expensive (but I can buy some of the components cheaply on eBay). This system recirculates the fumes, but drives out the grease centrifugally, and uses a charcoal filter and smoke filter-- both of which are about the size of two shoe boxes, so the overall effect is pretty good... at least, according to a few online reviews.

Who woulda thunk it would be so tricky to vent a range hood down through a basement?
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Old 01-12-2016, 07:25 PM   #9
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Originally Posted by Amazer98 View Post
I would avoid buying a fan blower for the hood, since that would just blow the air directly into the duct's initial 90-degree bend, creating a lot of back pressure.
I don't see the logic. Sure, moving air through a sharp 90 degree bend will restrict airflow. But I don't see how moving the blower location changes that. The bend is just as sharp, and if the blower is the same size, then it moves just as much air through that bend.

(However, it might be quieter with the blower in the basement)
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Old 01-12-2016, 07:53 PM   #10
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Originally Posted by SPS-1 View Post
I don't see the logic. Sure, moving air through a sharp 90 degree bend will restrict airflow. But I don't see how moving the blower location changes that. The bend is just as sharp, and if the blower is the same size, then it moves just as much air through that bend.

(However, it might be quieter with the blower in the basement)
Well, I agree the logic is a little elusive. As I visualize the installation of the hood with an integral fan blowing air directly into a vent that immediately turns downward, I see the air slamming into the back wall of the vent and being deflected back toward the hood with a lot of turbulence. Sure, some air will be forced down the vent, but a lot of static pressure will build up and the fan won't be efficient.

On the other hand, if the fan is located further down line in the basement, it can suck air down the ducts much more efficiently. At least it does so in my imagination... Does this not make any sense? The tech guy at Broan seemed to think it would work better, but he did stop short of actually endorsing any ducting down to the basement.
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Old 01-23-2016, 12:26 PM   #11
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I'm not that familiar with the range hood venting, but I assume that 600 cfm means that is what the fan is capable of, but where the fan is. As you get further away from the fan, the cfm decreases. You need to find out what kind of cfm you would have at the hood if the fan itself was 10' away with bends and given duct size, and the loss after the fan. I have Whirlpool, about 230 cfm, and it works well enough. Smoke from the fried steaks is about 80% vented now - it does through about 6' of 5" duct that came with the house, but 5" is not adequate.
I found the Broan tech dept is not that well informed, I think more a handy guy who is reading off the charts. Given Broan's $200 range hood is not really engineered - just decoratively designed - I have low expectations from Broan.
Try Fantech. They make higher quality inline blowers and have charts about the cfm, duct size, and loss of cfm. It feels intuitively that the loss of cfm would apply to front as well. Their tech may be able to help you better.

Last edited by carpdad; 01-23-2016 at 12:29 PM.
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Old 01-23-2016, 03:49 PM   #12
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Originally Posted by carpdad View Post
I'm not that familiar with the range hood venting, but I assume that 600 cfm means that is what the fan is capable of, but where the fan is. As you get further away from the fan, the cfm decreases. You need to find out what kind of cfm you would have at the hood if the fan itself was 10' away with bends and given duct size, and the loss after the fan. I have Whirlpool, about 230 cfm, and it works well enough. Smoke from the fried steaks is about 80% vented now - it does through about 6' of 5" duct that came with the house, but 5" is not adequate.
I found the Broan tech dept is not that well informed, I think more a handy guy who is reading off the charts. Given Broan's $200 range hood is not really engineered - just decoratively designed - I have low expectations from Broan.
Try Fantech. They make higher quality inline blowers and have charts about the cfm, duct size, and loss of cfm. It feels intuitively that the loss of cfm would apply to front as well. Their tech may be able to help you better.
I received the Broan fan by UPS from build.com, but had toi return it. The unit was bigger than I had visualized-- almost the size of a carry-on roller suitcase. The transitions ducts that came with it were also enormous-- they transitioned from the box inlets(maybe 6" x 18") to 10" round. I would have to further reduce it to 6" round to get the ducting through my sill... and there just wasn't enough room to do that transition.

I have since found a 600 cfm blower by Bosch, which is mounted in a much smaller box and is set up to take 6" ducting. This will greatly simplify the installation. I called Bosch cust svc with a few simple questions-- they are completely non-technical people there who can at best read copy directly from a product sheet (if you are lucky). They further claim that they are unable to put customers in touch with anyone who knows the product, just as a product manager or technical support person.

If nothing else, i have learned NEVER to order anything from Bosch... but I might break that rule if we go ahead with this basement installation. My contractor is trying to figure out if we can route ducting laterally across the tops of the cabinets and then turn through the wall, which would be better than sucking the fumes downward.

I've looked at Fantech's products, but they seem insubstantial compared to the Bosch and Broan fans, which are built for this specific purpose. But thanks for making the effort!
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Old 01-23-2016, 10:11 PM   #13
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[QUOTE=Amazer98;292736 My contractor is trying to figure out if we can route ducting laterally across the tops of the cabinets and then turn through the wall, which would be better than sucking the fumes downward.![/QUOTE]

That's exactly the way I did mine. I have a Broan Allure range hood. From the top of the unit I started with a short piece of 7" round, going into the short cabinet above. That goes into a 7" to 3 1/4" X 10" transition boot with a 3 1/4" X 10" 90 degree boot on top of that. I continued with 3 1/4" X 10" duct work across the top of the cabinets about 5' and then thought the outside wall with a termination cap on the outside with a spring loaded damper. I trimmed the 1" sides of the cabinets on top that stuck up to allow the duct to sit on the actual cabinet top. After the crown mold was installed, you can't see the duct. I have open cabinet tops due to a cathedral ceiling. Luckily the studs in the outside wall fell just right so going through was pretty easy. The hood works well and if I'm in the back yard on the deck in the good weather, I can smell what's cooking.
Mike Hawkins

Last edited by firehawkmph; 01-23-2016 at 10:14 PM.
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