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vent hood 911!

Posted by bicyclegirl (My Page) on
Mon, Dec 16, 13 at 14:23

Bad planning on my end. I need some quick suggestions on a vent hood insert. I've been putting off researching the vent hood since everything else seemed to be a bit more important & honestly, the vent hood wasn't as fun to research since it's going to be an insert to a custom hood! That was fun, but the insert...not! Now, I'm down to the wire w/ my electrician - no pun intended - who is coming tomorrow to put everything in the walls. He needs to know what hood I'm going to be doing so he has the specs.

Here's some info that will help. I have a 30" Wolf dual & my kitchen's not very big (appx 9x11), so was hoping to have my vent hood 32" W & 24" D. Does that sound ok for a 30" range? The burners are about an inch, maybe 2 from the sides of range. Thoughts on hood width & what the normal depth on a 24" depth range is. After deciding size, can you please give me your recommendations of brand for an insert? I really want something that's going to be quiet. I'd like to stay at $1000 or less, if doable. I've heard I need 1200 CFM's. Would 900 be ok for my Wolf?

Thank you in advance for any help you can send my way.

Follow-Up Postings:

RE: vent hood 911!

600cfm is plenty of draw for a 30" gas range against a wall so long as there isn't some bottleneck in the ducting or something that's constraining the performance. And if there is a bottleneck that's limiting the draw of your hood, upsizing the hood is a terribly inneficient and noisy solution.

How big is the duct (diameter in inches)?
How long is the run to the exterior vent?
How many turns are there along the way?

That being said, you don't need to select a hood in order to do the electrical work for your hood. More important at this point is to make sure you have sufficient framing in the wall, behind the sheetrock, to facilitate mounting whatever hood you ultimately select. Regardless of hood choice, the electrical work at this point is just gonna result in a hot romex sticking out of the wall, typically on the same circuit as the range.

RE: vent hood 911!

Gaucho, thanks for the info. I'm going to sound completely illiterate on this...because I am! Like I said earlier, I've done zero research on this one item. Does the vent hood not recommend the diameter of the duct? My hood will vent out thru the roof. There's venting there already that we were going to take this thru. However, there will be 2 turns to connect to the old one, both about 60 degrees. Will that be too much for the turns? From the kitchen ceiling, the run is maybe 4'-5'. I appreciate your thoughts on this. We were hoping to utilize the original venting to save money. But, if this ends up costing us more in the long run, we may have to rethink this.

Thank you for your help.

RE: vent hood 911!

As is prone to teenagers,"should-a, would-a, could-a."

The exhaust orifice in the hood is determined by the size of the CFM of fan assembly.

While turns and length can reduce the fan effectiveness, reducing the duct size will render it useless.

RE: vent hood 911!

One thing to be aware of is that it's OK to "step up" from a smaller orifice on the hood to a bigger duct, but not OK the other way around. For example, it's OK to go from a 6" hood orifice to a 7" or 8" duct, but not OK to go from an 8" hood orifice to a 6" duct.

The routing of your ducting sounds very good - the run is short and two 60 degree turns is not bad (60 degree turns are much better than 90 degree turns, so you can give a high five to whoever did that for you).

The most common duct size is 6" and most 600cfm hoods are configured with 6" orifices. This is the most common configuration and you'll have the most options in this configuration. So if you have a 6" duct, then a 600cfm hood with a 6" orifice is a good choice for you.

7" ducting is also common. Hoods with 7" orifices are relatively rare, but you can mate a 6" hood orifice with a 7" duct. So if you have a 7" duct you'll probably end up getting a hood with a 6" orifice and "step up" to 7" at the wall, which is a good arrangement. 7" duct probably isn't big enough to get you higher than 600cfm though. There are some hoods with 6" orifices that claim up to 900cfm, and this would probably work OK with a 7" duct, but I don't think it makes sense to use a 900cfm hood with a 6" duct.

8" ducting also available, and most hoods in the 900cfm range are configured with 8" orifices. So if you have an 8" duct then you could go this route. I think it's overkill though.

1200cfm is enormous and typically requires a 10"+ duct. I can't imagine using a 1200cfm hood for anything less than a 48"+ hood against a wall or perhaps a 36"+ hood over an island (extracting exhaust over an island is inherently more difficult), but even then I'd question the need to draw so much air.

Another issue with oversized exhaust hoods is that they can create pressure imbalances in your home than can cause bad problems - see link.

Here is a link that might be useful: Makeup Air

RE: vent hood 911!

Gaucho I have a stupid question to ask. Can a 900cmf island range hood with 6" hood orifices be configured with a 8" duct? I ask because we are putting in a new roof and duct. Wondering what size duct I should do to be on the safe side.

RE: vent hood 911!

> Can a 900cmf island range hood with 6" hood orifices be configured with a 8" duct?

Sure. Like I said, it's OK to "step-up" to a larger duct, but not vice versa.

If you're up for some additional detail ... it's best to step up as gradually as possible because bigger discontinuities result in more turbulence, and turbulence results in pressure drop (pressure drop is the standard metric of duct performance, big pressure drop = bottleneck). So, for example, it's better to step up gradually from 6" to 7" and then from 7" to 8" than to step up from 6" to 8" all at once because the 6" to 8" step will result in more turbulance (less laminar airflow) and more pressure drop than two smaller step-ups. This really is splitting hairs though, so don't get hung up on it.

And while we're at it... if you have a turn and a step-up, it's best to combine them if possible because the pressure drop that results from the combination of the two is almost always less than the pressure drop of the two in series.

This post was edited by GauchoGordo1993 on Tue, Dec 17, 13 at 22:22

RE: vent hood 911!

" jterrilynn"
You may find a single piece 6" to 8" adaptor, or a sheet metal shop can make one up for you. The key ius to have the 6" male and the 8" female so that flow is not restricted.

In the 35+ yrs. of remodeling, I never heard of a 6" vent, until I began posting on these forums.
Microwaves and squirrel cage fans all had a 3-1/2" x 10"
vent adapter, which is a standard rectangle vent, or were adapted through a transition to a 7" round, which fit snuggly into the existing roof jack.
Vent appliances with 7" round vent adapters were powered by a propeller style fan.

As the industry and the engineering have evolved, the only standardization is with the mfg.

The key to success is to not reduce the volumetric efficiency, IE, do the math.


RE: vent hood 911!

Awesome info Gaucho! Thanks so much for the explanation. Very understandable!

Those 2 60 degree turns aren't done yet. My AC vent guy is going to do that when the hood is installed. My house is very small & very old. I had central Air & Heat installed about 12 yrs ago, so he's going to tie in with the venting above the kitchen that goes out the roof. My stove is just a bit off center from that, which is why he has to go over some. Not sure how big my ducting is, but I'll ck it out & guess I'll be looking for a hood w/ 600 cfm.
Also appreciate the link for makeup air. I'll take a look at it.

Thanks again for your help.

RE: vent hood 911!

Good call on the 3 1/2 x10" rectangular snoobyb - I forgot about that one, which is funny because I just removed one from my wall a couple months ago.

Since the capacity of a duct is proportional to its cross sectional area, it's true that a 3.5"x10" rectangular duct is comparable to a 7" round duct. The cross sectional area of 3.5"x10" rectangular duct is 35 square inches, and cross sectional area of 7" round duct is pi*radius^2 = 3.14 * 3.5^2 = 38.5 square inches.

RE: vent hood 911!

Thank you Gaucho!

RE: vent hood 911!

Kitchen venting appliances have long been an "interesting"
From the natural vent screen, to the municipalities who require a 42" venting appliance for a 36" gas range top and make-up air, to "we don't need it", it's an unnecessary expense and the venting takes up cabinet space.

While round duct certainly takes up more space, the fittings for the rectangular are more expensive, and you'd be surprised if you could find a kitchen designer, that had a clue.

RE: vent hood 911!

Gauchogordo1993-thanks for the link. That was the best article on make-up air that I've ever seen. I finally 'get' it.

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