Bad idea - you will reduce the effectiveness of the hood as well as increase noise. Change the ducting or get a different hood. Check out Kobe hoods - you might find one which meets your needs. Since you have a tough inspector, they may well require make-up air for a 1200 CFM hood - you should find that out as soon as possible.

I did a quick check of the Kobe site - they make 1000 CFM hoods that work with an 8" duct. Your VAH is only 900 CFM - that equivalency stuff is marketing hype.

Thanks, Weissman. Unfortunately for my situation, the Precision users manual states that a 1200 cfm hood is recommended. I'm doubting there are any 1200 cfm hoods that will work with an 8 inch duct but haven't done that much research yet.

Are you getting a grill on your rangetop? The Capital manual states that you only need the maximum 1200 CFMs if you're using a grill.

What's the reason for reducing to 8"? I'd also like to know what evidence the above poster can produce that the VH equivalency is "marketing hype."

The evidence is that CFMs are CFMs - if a VAH is rated at 900 CFMs that's its rating - it won't operate at 1350 CFMs. Their "claim" is that other brands rated at 900 CFMs won't actually produce 900 CFMs and that it will take a hood rated at 1350 CFMs to actually move as much air as a 900 CFM VAH.

Time to find a new HVAC guy in my opinion. 10" to 8" for the majority of the run will kill the flow and performance.

Here's the key giveaway: "BUT should work fine. " is he willing to bet the fee (parts and labor) that it will work? Better to have a proper design from the start, not a back of the napkin guess - especially one that 's errs toward the low side.

If he were suggesting the opposite then I might get on board. 10" for the majority of the run reduced down to 8" for a short "drop" would be a more appropriate way to design a duct system. He is correct about the noise, and weissman is correct about CFM being CFM.

BTW - what is a pretty good price???

No grill. The rangetop has 4 burners and a griddle. Will I be fine with the 600 CFM model based on that?

Just looked again at the Capital manual and kudos to Weissman.....

It states "Normally the maximum blower speed is only required when using the grill section"

Also the CFM rating is only a "recommendation". Surely the inspector would read this and be good with the 600 CFM hood since we won't have a grill.

600 CFM should be more than adequate - probably won't need makeup air unless you live in a northern state that requires it. Your big issue would be convincing your inspector.

600 CFM is probably enough given the short relatively straight run, BUT it may need to be run on high much of the time which will be louder than a larger motor running at a slower speed.

My suggestion would be to get larger than 600cfm (900?) and run it at a medium speed.

ditto every word from weedmeister, weissman, antss.

a runs of 25 feet is not considered short, in certain circles. Its resistance to flow is increased a great amount compared to a 25 foot run of a larger diameter.

but 1200 CFM is not necessarily required, recommended or wise.

Mention the climate you live in. If you know where air slide into your house normally, mention this too. Most houses are leaky.

I live in the central part of the US. Pretty mild climate. The house is new construction so not sure where air "slides in".

It is possible to measure a fan by itself, or a fan and a hood together, and determine a volumetric flow rate (cfm). This cfm only applies to the zero static pressure case of the measurement. Because the VAH fan is part of the hood assembly, their 900 cfm rating MAY apply to the combination.

When duct transitions and long ducts and negative house pressures are involved, the achieved volumetric flow rate will be less. Without knowing all the pressure losses as a function of cfm, AND the fan's cfm performance as a function of the pressure loss (fan curve), the actual cfm that will be achieved is not knowable.

The AHJ will probably not do an analysis for you based on unpublished information. If the rating of the hood is at least equal to the requirement of the rangetop, he will likely be satisfied. However, he may insist that the duct diameter conform to whatever requirements VAH or other hood manufacturer specifies.

The choice with a tough inspection venue would seem to be between the no argument fully compliant in specifications approach, and a complicated analysis approach that the AHJ would likely only accept if the analyst had suitable credentials.

kas

25' + 45 degree turn is not a long run. It's about 35-40' in equivalent lengths of straight run.
600 CFM for a 4 burner top is fine, therefore 900 CFM is more than you need. 1200 CFM is insane
8" duct for a 600 CFM hood is fine. 10" is better.
Transitioning from 10" to 8" is fine.
Just getting from the hood to 10" duct requires a transition included with the hood, and transitioning between duct sizes is the rule not the exception in HVAC systems, though should be minimized.

Though it seems this question has been answered, you should have no problem getting a letter from Capital approving a 600CFM hood over your 4 burner 36", if your builder inspector requires it.

Be aware of the lower speeds of your blower which you will be using more often than the upper end. Is the lower end more or less than 300 CFM. 300 is too high for a minimum. How many speeds do you have? Max power usage is infrequent, though honestly, without max power, my mother-in-laws cooking was a constant threat to my eyesight...so much garlic fried in sesame oil, I could hardly see until we upgraded.

Here is a link that might be useful: ApplianceAdvisor.com

advisor - while transitions are common practice in the HVAC industry , this guy's design has whatever size takeoff VAH is supplying going to 10" then choking down to 8" again somewhere near the exit point. Big belly in the middle is hardly good design, or use of materials!

Disagree with you that 1200 is "insane" especially if it's a remote blower. It's probably overkill for most cooking situations with that top but it will allow for running at slower speeds with less noise that way. But that's different than insane. 1000cfm would prob be a better choice given the economics. But now we're getting into a complex discussion of fluid dynamics + econimics which I'm guessing few understand or even want to talk about.

Also, the BEST inserts we use a lot have a 10" takeoff at at the top, so don't need a reducer. Granted , this is not typical though.

Our rule of thumb with a pro range top is 900- 1000 cfm minimum and we strongly rec. remote or inline blowers.

all good points. But how will the lower range of CFM needs be handled? A three speed switch is common and it's robust technology. It is not appropriate for a fan that is as large as 1200 CFM. A variable speed controller (like a dimmer but for motors) might cause a lot of hum at lower speeds. Some motors might not pull well when they are spinning at low speeds, so there might not be much choice but to have a lot of air exhausting...

david - it's not so much dependent on the switch as much as the fan design and it's enclosure. A fan curve can tell you a bit of of the story. An any reostat hum will be drowned out by the moving air, especially on a remote blower.

antss: it looks to me like the transition to 8" is very near the hood, not near the exit, unless I mis-understand your comment. If there's concern, then a transition from hood to 8" can be fabricated by a hvac contractor....I just think it's un-necessary.

As for the blower, it also does not make sense to me to install a blower twice as powerful as necessary in order to have a single useful quiet speed. A more sensible 600 cfm blower will have 3 useful speeds, none of which will be too powerful or noisy, or result in nasty make-up air issues.

Here is a link that might be useful: ApplianceAdvisor.com

advisor - maybe we just are reading this differently:

"contractor is saying we can run 10 inch off the hood and then "choke it back" to the 8 inch for the majority of the run. " You see , only an idiot would got to the trouble to install an 8" to 10" transition at the hood and then immediately add another one going in the opposite direction to go back down to 8". So what is likely happening is there is the takeoff at the hood at whatever size to 10" for a run of between 2 and 24 feet at which point there is to be another reducer of 10" down to 8" for the rest of the run - or majority as o.p. says. This leaves a big belly in the middle of the assembly.

If you think this is good design then you are certainly entitled to your opinion, but fancy website aside I say you're wrong. You are correct about the transitions 8" to 10" is quite common , you can get one of the shelf at your local home depot, and VAH will even sell you one. They'll even sell you one of their wacky 8" plus 6" dual takeoff to 10" round if you get one of their larger hoods. BUT, I'm not letting any contractor go from 8" to 10" back down to 8" again in our projects. You can do what you want, and I'd suggest mrd gets someone that knows what they are doing locally to advise him/her because 1. neither you or I are on site to see what's actually going on, 2. have not been engaged to design the system, 3. seems that the onsite HVAC guy is not quite on the top of his game based on the quoted gameplan.

RE: the 600 cfm, might be enough, but what if it's actually not in this case. Are you going to pay for the swap, refund the consult fee, ect.??? I know 900 + will work because it there is a lot of xtra headroom built in. It might cost more, or maybe not as VAH is proud of their hoods, but that cushion is nice. You also do not mention how loud that VAH is going to be especially on full tilt ! May not be important to mrd as (s)he didn't mention it - but what if it is???? Doesn't seem prudent to make a rec to a stranger that is too conservative - but hey it's a free country.

Appreciate all the feedback from this thread. My builder and I have decided to use the 8 inch duct that is in place and install the 600 CFM VAH insert.

Honestly I can't imagine a time when I would be using more than two of the burners on high anyway. Even if I am using the wok and a couple other burners at medium heat, it seems to me this hood will work plenty well if my "catch area" is designed well with the hood overlapping the cooktop at the counter's edge.

I know it would be a "nice to have" on the higher CFM hood but it is not worth the trouble to redesign the ductwork at this stage based on my anticipated cooking habits.

Thoughts or opinions welcomed. Also I would be interested in what meals people typically cook where they are using 3 or 4 burners on full power.

advisor - check the VAH specs before you start giving advice-which models have a 10" takeoff? None that I know of, So, a fellow (op's hvac guy) that is trying to re-use existing ductwork is unlikely to scrap the factory takeoff in favor of spending time fashioning his own! What part of "choking down to 8" for the majority of the run" don't you understand - or - think is good design ??? We don't even know what model the op is using but I'll bet your lobster dinner that none of their 600 cfm units have larger than a 8" takeoff.

Broad statements and assumptions are great until you run into a situation where they don't apply - like this one.

Here's what's really germain to the O.P. - neither of us are on site or know the specs to design a system for him/her. It seems clear that the HVAC guy is trying to get by with as little work and expense as possible. Inspector is a stickler for details and HVAC guy is trying to do the bare minimum in order to pacify inspector. Show him 10" duct till he can't see any more and then just use whatever is there and can't be seen. This may pass inspection, but is not what VAH had in mind or is good for performance.

And that's my point. If you want to continue to debate broad concepts thats' fine too. At any rate, even this is moot in this case because the folks onsite have determined that 8" the ENTIRE length is the way to go. I don't disagree with this for 600 cfm.

If I understand your terminology, the VAH 900 CFM blower comes with a transition to 10" round. (the takeoff?)
It transitions three blowers of 300 CFM each, to two hood exits of 8" & 6" before entering a transition to 10" round.

Or at least this is what VAH told me.

Here is a link that might be useful: ApplianceAdvisor.com