Very Specific Hood Query - please help!!

SherylJMay 20, 2014

We are installing a 36" Capital Culinarian AG range with four burners and a grill in an island. (I know it would be better against a wall, but it's impossible). We had planned on installing a 42" hood with an internal blower running 900 - 1200 CFM's. It will be about 30" above the range top and vents directly up through the roof (which is sloping but at the very highest point, just about 9'. The problem is that our existing duct is 8" and we've been told you can't run anything more powerful than 700 CFM's through an 8" duct.
Our house is quite old with structural beams in the pitched ceiling that are no way up to present day code. If we enlarge the existing duct to the 10" that would be required for the higher CFM, we'll be cutting into beams and opening up a whole can of worms with inspectors. Our contractor is advising (pleading/warning?) us to stay with the 8" duct.
So my question to the experts out there is will 700 CFM's be enough? It doesn't have to be perfect but it has to be functional enough to be able to use the grill. Or is there another possibility? Can you run something more powerful through the 8" duct? If so, any brand suggestions would be much appreciated.

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Two possible options

1) Change the shape of the duct to 8" x 12"
2) Use a remote blower on the roof rated at 1000 cfm

    Bookmark   May 20, 2014 at 6:12PM
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Thanks, Trevor. One question, which may be stupid, but I am new to this... Why would the external blower at 1000 CFM work better still sucking through an 8" duct than if it was an internal blower? I've been under the impression that 8" is just too narrow to really allow that much CFM. Am I wrong? : )

    Bookmark   May 20, 2014 at 7:16PM
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It is my understanding that sucking through an 8" duct works more effectively than blowing

    Bookmark   May 20, 2014 at 7:34PM
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Got it. That does make sense.

I'm restraining myself from a NSFW joke here...

    Bookmark   May 20, 2014 at 8:02PM
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In theory, a given blower yields a given flow rate determined by the pressure loss, and this is independent of whether the pressure loss is behind or ahead of the fan. Variations might exist due to subtle effects determined by blade shape, but I wouldn't assert that there would be a material difference.

However, with a roof fan there are options for blowers that may not be available within a given hood line. In such a case, the blower is selected to provide the required flow rate at the higher pressure drop of the smaller duct. (This may be noisy.)

Better, though, is to use a duct configuration, such as rectangular or elliptical, that conforms to the beam configuration and has enough sectional area that a conventional hood or roof blower can be used.


    Bookmark   May 20, 2014 at 10:22PM
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So, post 2 by Trevor is correct, but post 4 is suspect. The 1000 cfm at the roof with an 8-inch duct may provide the same flow rate performance as the 700 cfm in the hood with a 10-inch duct because at the different pressure drops both actually flow 500 cfm.


    Bookmark   May 20, 2014 at 10:28PM
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FWIW, I have a very similar set up... Bluestar 4 burner with griddle under a 36" x 27" Prizer island hood. 1000 cfm remote blower mounted fairly close above our sloped ceiling with 8"duct. Hood is 36" above range top, and it functions fine.

    Bookmark   May 20, 2014 at 11:02PM
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I was so hoping you'd weigh in, Kas! Here's my next question. If the 1000cfm hood at the roof with an 8" duct may perform at the same rate as the 700 cfm in the hood with a 10" duct, where would you rate a 1000 cfm in the hood with an 8" duct? I think we're stuck with the 8", so I'm trying to figure out if we will see any measurable performance difference with 1000 cfm vs. 700 (both internal). And then I am trying to decide if this will be enough performance for the Capital range we have already purchased
I should add that I have been cooking in this kitchen, unremodeled, for 20 years, with a professional US Range, not approved for residential use, and with no hood, just a paltry air blower/fan that does pretty much nothing. I've learned what I can cook and what I can't, or rather, what happens when you have no ventilation... So anything will be an improvement! However, we are spending a lot on this remodel and have purchased a powerful range because we love to cook, so we want to be able to use it without smoking up the house!
I guess what I'm trying to say is that we don't have unreasonable expectations of a ventilation system, but we do want to be able to use what we've bought without smelling dinner for the next three days and having watering eyes while cooking (both of which we're accustomed to with our old kitchen).

    Bookmark   May 20, 2014 at 11:05PM
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SherylJ wrote: "If the 1000cfm hood [blower] at the roof with an 8" duct may perform at the same rate as the 700 cfm [blower] in the hood with a 10" duct, where would you rate a 1000 cfm [blower] in the hood with an 8" duct?"

I'm tempted to write: loud. But using the pressure drop argument that I wrote above, a 1000 cfm rated (at zero static pressure) blower in the hood with any particular duct should behave the same as the same 1000 crm blower on the roof. Of course, blowers designed for the different locations may well be different, and without the fan curves (flow vs. pressure loss) for each a better answer is not possible.

May I suggest that if the duct path allows, a tiny part of the "lot" used to remodel be spent on a roof blower. It will be quieter. And seriously consider Trevor's suggestion of 8 x 12 rectangular ducting. It may look better unless your 8-inch duct is already nicely enclosed.

If your roof blower comes from Wolf (actually Broan) or Broan, you can get fan curves for the unit. Other sources such as Fantech or Abakka may also supply curves. Assume 0.1 inch of w.c. loss for baffles and use an on-line calculator to find the pressure drop in your ducting, whatever it is. Then you can use the fan curve to find the actual flow. (Note, baffle loss depends on flow rate just as duct loss, but practically no one publishes baffle loss curves.)

Aim for an actual flow of at least 90 cfm per square foot of hood aperture area. If this area is 42 by 24 inches, for example, then the area is 2 x 3.5 = 7 square feet, leading to a need for an actual flow rate of 90 x 7 = 630 cfm. A 1000 cfm blower with the losses that are likely will probably meet that criterion.

You may not have room for a silencer (muffler), but most of the time you will be running at a much lower fan speed than max and the noise should be non-intrusive.

Flow rates above what your house leakage can support without back-drafting combustion appliances require some make-up air (MUA) scheme. I won't address that here, but please review the scores of relevant threads here on the topic.


    Bookmark   May 21, 2014 at 10:27AM
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