I haven't searched for them, but I'm pretty sure you can get a reducer to go from say an 8" duct to a 6" hood opening. Just to be clear, it's not just the diameter of the roof cap, it's the diameter of the entire length of the duct.

Not too many hoods accept a 6" duct so if you have the opportunity now, you should put in at least an 8" duct. That will give you more flexibility in the future if you need to change hoods. Again, it's not just changing the duct cap - it would entail replacing the entire duct.

Thanks---when I searched for reducer, I found a bunch of galvanized duct reducers on hydroculture sites (??)---is that the same piece I'd use here? Also, to further complicate things---the vent cover for this hood is 7 7/8 deep. So I assume that means trouble if we go with an 8" duct....is 7" going to do much in terms of adding future hood options, or do we really have to go to 8" to open up the options? Too many decisions....

A Master distributor is the firm that represents a manufacturer like Kobe in the US. They have a web site, a phone number, and an email address. They always have technical support people. At least one person is high level with loads of experience with real life installations. After you get information from them post here to connect the dots.

Increasing the diameter of a duct from 6" to 8" allows the air to move half as fast over that section of 8" diameter, for that length. This is because the volume inside an 8" round duct is double that of a 6" round duct. (Pi times R squared, compared to Pi times R squared, for each duct, means you only need to compare the R squared, and a ratio of 36:64 or 9:16 is about half::double)

As far as I know, no harm is done when a run of ducting affords less friction to the air being moved. Friction occurs because the air stream rubs against the sides, and this portion of the air stream next to the duct wall cannot keep up with the rest of the air stream, so it is slower, and it can also eddy into the middle and make the air stream less that optimal. In a lab where they test the product for regulatory approvals, they never test real life performance with a real life typical run of duct with a roof cap. Manufacturers can make any claims they want about performance and noise, which maybe don't occur in anyone's real life installations. Consumer Reports doesn't install real life simulations of ducting either; CR only looks at the product in the box and not a real install. One day this weird situation may change for the better, but not this year. If we just required manufacturers to publish Their Own Real Life Simulation test results, we would be far far ahead, and it wouldn't cost more in independent testing. Luckily, we live in a big world and as luck would have it, in other countries they study noise as part of the specs, and they do focus on performance of the product-installed-in-its-system not the product alone. By the way, turbulence = noise = much less than optimal flow.

Increasing the diameter of a duct from 6" to 8" allows the air to move half as fast over that section of 8" diameter, for that length. YOu may then ask if the restriction at the termination, the roof opening, will make it all meaningless, or if (the opposite case) widening the duct will be worth it. The answer is more the latter. A single restriction (where the opening is smaller than the rest) does not negate the beneficial effects of widening the entire duct. Key words to go web searching with are "Venturi Effect" if you want to read up on this. The air at a pinch point moves faster. Far far faster than could be sustained over a longer stretch.

More posts may be written, showing you how to read the technical specs of the fan and telling you more about noise, turbulence, flow and so on. Before assuming you know more, and before you decide to do anything, prepare a single question to ask Kobe. Only they can speak about their product. Whether it will improve ducting performance if you widen the duct. That answer is either "Yes" or "Yes, but..." and Kobe is the source of that answer.

hth

The only comment I would add to Davidro1's message is that slower air velocity eventually leads to increased grease precipitation on the duct interior surfaces. I don't think the case discussed here has a problem with that, but it would be useful to check your ducting every year. It may be that six inches diameter is so restrictive for a claimed 720 cfm that increasing it to eight inches causes only a small decrease in velocity. Much more information would be needed to hazard a real guess.

kas

I posted this on the Kitchens version of this thread, but I did call Kobe. They said it shouldn't be a problem to go larger but that the duct cover would not cover an 8" duct (which I knew). The woman did not think it would have much of an impact positively or negatively on the performance of the hood, though.

kas - "slower air velocity eventually leads to increased grease precipitation on the duct interior surfaces"... That might be misleading. There are many factors at play which I'm not even close to being qualified to discuss, but I believe that in 2002 the NPFA decreased minimum velocity requirements after studies showed that higher velocity = higher turbulence = more grease ending up on the ductwork. 500 fpm is the lower requirement I believe. I'm not about to do the math right now, but given that Kobe notoriously specs smaller duct sizes as compared to other brands with supposedly the same CFMs, I'd not hesistate to bump it up. My completely unsubstantiateded guess is that they just enjoy marketing to the retrofit market.

Given that we're starting from scratch (replacing a 1950s fan that vents into the attic!)---are there other hoods you'd recommend? We need a 36" chimney wall-mount hood with at least 600 CFM, ideally with baffle filters and ideally with a quiet setting option, for under $1K. I haven't really found much else out there...

aremis - If you're directing that question to me, my answer is "No, because I don't know." What I do know is that historically I've seen folks here happy with Kobe which leads me to believe that they're not only attractive and quite reasonably priced, but also effective.

I'd probably bump up to 8" if I could, but I wouldn't lose sleep if I couldn't, esp given that Kobe is shrugging their shoulder at the benefit for this particular hood. I assume this remodel is expected to last you at least a decade or until you sell the house?

For sure! (The last remodel this kitchen saw was in 1939, so this one had *better* last at least a decade... ;) That's largely why I hadn't thought too hard about it---for a few hundred dollars, it would likely be a nonissue if it came time to replace the hood and the duct did need to be changed. Now, if having a different duct would make *this* hood perform differently/better, it might be another conversation, but I'm trying to avoid revisiting this particular decision if there's no compelling direction to go in.

In a quick search, the only hood I pulled up this time around that wasn't in play last time we looked (last winter) was the Aga Pro+, but that's still a couple hundred more than we'd like to spend---and it also uses a 6" round duct. (What's more---it includes a converter down to a 5" duct!) Ah, well.

Is this going to be a straight duct?

I'd be happy with a Kobe and a slightly larger duct. It is true that Kobe always seems to spec small ducts. Some other companies also do this, on some of their models.

Yes, straight and just a few feet past the ceiling---so very simple. I'm wondering if it even makes sense to do an 8" duct just for the last couple of feet given that we have to drop down to 6" anyway at the ceiling line, though.

I stand mostly corrected by foodonastump. According to page 31.7 of the 2003 ASHRAE Handbook - HVAC Applications:

'Model building codes, such as the IMC, and standards, such as NFPA Standard 96, have set minimum air velocity for exhaust ducts at 1500 fpm. ... ASHRAE research (Kuehn 1998) indicates that there is no basis for specifying 1500 fpm minimum duct velocity for commercial kitchen ventilation, but rather supports reducing NFPA Standard 96 recommendations to 500 fpm. This change allows flexibility for design of variable-speed exhaust systems and retrofitting older systems, though current design practice for new single-speed systems will generally continue to use design duct velocity around 1500 fpm.'

The point here is that at the minimum variable speed setting (for commercial kitchens), the flow velocity should be 500 fpm. Most residential variable speed systems will go down to practically zero fpm, so at those velocities, one would be advised to not be broiling hamburger.

Grease effluent consists of various sized particles and vapor. The vapor will not condense out until sufficiently cooled. The particles will either be affected by the centrifugal action of the baffles, or not. In the latter case I think the turbulence in the ducting would have to be extreme to compete with the baffles.

I don't have Kuehn's paper so subtleties and assumptions should be checked for relevancy to residential use. A synopsis of the paper's results are available on line. I still need to find time to digest it.

kas

Here is a link that might be useful: New Rules for Kitchen Exhaust

I didn't find the time in the last few minutes, but the last sentence is relevant: "Lowering velocity reduces grease deposition in virtually all cases when ductwork with insulation of R-10 or greater is used." My ducting is mostly in an attic not significantly warmer in the winter than the external environment, and it is not insulated. So YMMV.

kas