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Insert choice with 8" duct

Posted by Ott2 (My Page) on
Wed, Jul 24, 13 at 16:14

I am trying to funnel all I have read on ventilation and range hood inserts into a decision that best fits my situation. I don't want to mess this up, and would like to know how you would choose to ventilate in the same situation.

This is similar to a thread that new_morning started recently, except that I will likely be constrained to an 8" duct.

We will have a 42" hood over a 36" range top (BlueStar, 4 burner, griddle). I can make the capture area however deep I need it to be. Duct run is about 8 feet total: up about 3 feet, elbow through a wall, then up and out the roof. There is no place for an inline ventilator, and a remote does not seem be be an option at this point. So, internal ventilator is necessary. I know I will use the ventilation much more at lower speeds than at higher speeds, with occasional needs for significant ventilation. MUA is not a problem for us per GC, DH, HVAC guy, etc.

These are the insert models I am debating between, and the reps I refer to are with the respective companies:

Abbaka - The Abbaka rep recommends the XL pro hood liner with 600cfm and designed with 8" ducting. Rep said that their "good" 600cfm blower should handle my range top. They did NOT recommend transitioning the 10" duct on the 1200cfm insert to my 8" duct. The rep also indicated that they have two types of single blowers, and one is larger and more powerful than the other. They recommended the larger blower for my use. I requested information on the two single blowers so that I could see how they differ. Does this ring a bell for anyone?

Modernaire - PSL-240 - Modernaire rep also recommended using 600cfm, single blower, designed with 8" duct rather than using 1200cfm unit that would require transitioning a 10" unit to my 8" duct. Rep believes that the 600cfm w/ 8" duct will be fine IF my capture area is good. Rep reminded that if I can get 10" ducting, the 1200cfm unit would be preferable.

Prestige - UIB40-1200. This rep recommends the 1200cfm. On this unit, there would be two 6" ducts that open into a rectangle transition piece, and the transition piece would connect to my 8" duct. If I understood correctly, this process would be the same if connecting to a 10" duct. Rep said I might have a small increase in noise with 8" duct over 10" duct which I would expect. "Small" was not defined. :)

Should I do the Abbaka or Modernaire, single blower at 600cfm, with the 8" duct design, making sure I have a good capture area? If so, is one preferable over the other, and why?

Or, should I do the 1200cfm Prestige, with the two 6" ducts going into the rectangle transition, then into my 8" duct? Is the noise at lower speeds in this configuration going to be significantly louder than the units above making the extra cfms a poor trade-off?

Is there a better option that I am missing?

Thanks, Ott2


Follow-Up Postings:

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RE: Insert choice with 8" duct

Have you checked out the Zephyr lineup? I have the 36" Monsoon II DCBL series and I think it maxes out at 700+ CFM and I know it uses a 8" duct.

I have it over a BS 36" 6 burner range top and have never had an issue with it.

Phil


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RE: Insert choice with 8" duct

An actual 600 cfm through an 8-inch duct results in 1720 ft/min air flow, about the upper end of the velocity one wants to have to balance between keeping grease off of the straight duct and keeping grease off of the duct bends.

But then, none of these units will pull their rated air flow rate through the ducting, even with the house windows and doors open. In particular, trying to get 1200 cfm through an 8-inch duct will have enough pressure drop that the actual cfm achieved will be significantly reduced. The gain over the 600 cfm rated fan may be modest; it depends on the different fan curves.

These hood suppliers, upon learning your duct configuration, should have calculated for you the predicted cfm.

If I had to take a WAG at achieved cfm, I would expect the 600 fans in 8-inch duct to flow around 400 - 500 cfm, the 1200 fans in 10 inch duct to flow around 800 - 900 cfm, and 1200 fans in 8-inch duct to flow around 600 - 700 cfm. Again, fan curves rule, but hood filter pressure loss is also a biggie in this guess.

The only advantage that I can see with a 1200 cfm fan into 8-inch duct is that running the fan(s) at a speed to yield a nominal 600 cfm will potentially be quieter than a 600 cfm fan running full power. But this is only the blade tip noise. Duct noise and baffle or mesh noise will be the same. And attempting to run it at full power should be an interesting experience.

Abakka may have designed their larger fan to provide a more vertical fan curve such that the cfms will stay up for more restrictive ducting conditions. If so, that would be a direction I would go, given all of the requirements. Ask them what cfms you should get with their hood, your duct configuration, and each of the fans. Any answer will only apply to perfect make-up air, i.e., open windows and doors, but it will provide a baseline.

People in this business should have software to do these calculations, and they will know the unfortunately proprietary pressure loss properites of their hoods, and their fan curves.

kas


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RE: Insert choice with 8" duct

I have a Blue Star 42" RNB with grill. I have a 42" Modernaire PS-11 which is 24" deep. We also have an 8 foot or so run, with one turn and then out the exterior wall. We have a 1200 cfm blower.

During construction but after ordering the hood we discovered that we didn't have room for a 10" duct so we had to go with 8". It works perfectly fine even when we grill. It is noisy but gets the job done.


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RE: Insert choice with 8" duct

Thank you for your replies!

philwojo - Thanks for the Zephyr suggestion. I didn't realize that the DCBL model used an 8" duct. I will look into this option. A Zephyr model would be the easiest and least expensive route because they folks building my house are most comfortable with Zephyr and I can get a good price on it with my appliance package.

Kas -
"If I had to take a WAG at achieved cfm, I would expect the 600 fans in 8-inch duct to flow around 400 - 500 cfm, the 1200 fans in 10 inch duct to flow around 800 - 900 cfm, and 1200 fans in 8-inch duct to flow around 600 - 700 cfm."
I have never actually had real ventilation. Please forgive what is probably a "duh" question, but I am requesting one more WAG from you! Assuming the duct configuration above, if I were to fry bacon for my meat-loving son, would the 400-500 cfm I might get with a 600 fan be enough to remove the resultant odors? Or if I blacken chicken breasts, would the 600 fan suck up the smoke? For basic tasks like these, would I get better performance with a set-up like nycbluedevil's, or for these basic tasks will the performance be about the same? What kind of task am I doing if I start seeing the benefit of the extra cfm I might get with a 1200 cfm fan with an 8" duct similar to nycbluedevil's set-up?
And, thank you for suggesting questions I can ask the manufacturers to try to get a better idea of the real cfm I can expect for my configuration. I'll work on that today.

nycbluedevil - Thanks for your feedback on the Modernaire over BlueStar mix with 8" ducting on an 8 foot run. Almost apples to apples for me. Do you find the set-up particularly noisy for daily tasks as well, or mostly just when you are increasing ventilation for tasks like grilling?

It doesn't seem that there is as much info on gardenweb for Abbaka relative to Modernaire/Prestige/Zephyr. I've got a solid price quote on Abbaka in case I choose to go that route. How does Abbaka quality/performance/service compare?

Thanks so much for your help! Ott2


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RE: Insert choice with 8" duct

Here is a link to the Zephyr page:

Zephyr Monsoon DCBL LINK

On there you will see it says this:
"Ultra-quiet 715-CFM motor @ 5.8 sones on max"

There is other information as well, here is the part# for the 42" model - AK9440AS

That is listed under the "features" section.

Best of luck, and KAS is a great help in this area without a doubt.

Phil


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RE: Insert choice with 8" duct

I have a 36" Blustar with griddle, and I too had roof framing constraints that limited me to a 8" duct. I went with a 1000cfm remote blower, however it is only about three feet from the hood, so the sound is probably not much less than an internal fan. In my search for my 36"x27" Island hood, I had it narrowed down to Modern Aire and Prizer. I decided on the Prizer, and am very pleased with both it's looks and function, but I don't think you can go wrong with Modern Aire.


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RE: Insert choice with 8" duct

First, the hood aperture has to overlap the expanding area of the rising pan effluent to capture it all. Second, the hood air flow velocity must be high enough for containment; I use 1.5 ft/s (90 ft/min) as a goal. So, the cfms should be sufficient for the hood if the aperture area in square feet times 90 equals (more or less) the cfm capability. Not sure what your aperture area actually is for this question.

Oh, also, while a hood might be described as sucking up the smoke that enters it, the pull any distance from the hood aperture will be very weak. Hoods actually depend on the rising effluent velocity to get the grease, water vapor, and odor to them. In other words, the perfume from big pile of cold cilantro on the cooktop can only be removed (assuming one wanted to) by cycling all the room air out of the kitchen and replacing it. It won't conveniently rise into the hood.

kas


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RE: Insert choice with 8" duct

Ott

I run the vent when I am grilling or sautéing. I use high or close to It and it is definitely noisy. Not like Vita Mix noisy--we can carry on a conversation but happy to turn the vent off as soon as possible. We use the vent, if at all, on a low setting when boiling water, making rice, etc. The noise is there but tolerable.

I am sure Kas is right and that the hood sucks with less force (I think that is what he is saying) because of the smaller duct. But it works. Bottom line, my baffles are greasy but my cabinets are clean. Don't let the perfect be the enemy of the good.


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RE: Insert choice with 8" duct

I was actually saying that the air velocity directed toward the hood aperture at any distance from the hood aperture (to the side or down) is very low relative to that at the aperture itself. My immediately previous comments did not directly relate to the duct size, the second paragraph related only to the widely held belief that hoods suck up cooking odor from the cooktop.

kas


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