Yikes! VaH is claiming that one fan is OK with a seven-inch duct, but with two fans you will get smoke curling out of the hood? Must be a magical property of magic lungs.

We need to know the aperture size of your prospective hood; that is, the dimensions of the hood base where air flows in without counting lip area, switch area, lamp area, etc.

VaH fans have fan curves that have greater flow rate drop with pressure loss than some other blowers, so with a restricted duct one might expect lower flow with their blowers than just adding two blower flows together, but total flow has to be higher than with only one [same type] blower.

Suggest you review past-year topics here with the key words "fan curve."

Note that number of burners used is not a first order factor in fan flow rate needed. The effluent rises to the hood with some velocity and has to be captured and contained (kept from reflecting out). This requires comparable air velocity where the smoke hits the sheet metal. We don't get to direct where the blower pulls from, so the minimal air flow velocity (proportional to cooking temperature) has to be provided across the entire aperture.

Pressure loss in an 8-inch duct transitioning to 7 inches at the roof will be lower than for an entire 7-inch duct. Transition piece should be designed for minimal turbulence (and hence pressure loss).

kas

Thank you, Kas for your response. I feel like I need to be an engineer to figure this out.

XRH9 Excalibur - UNDER CABINET - this is the hood, but I haven't purchased it yet, so I can't find on their spec sheet the exact dimensions you asked for. It is 36 inches by 21 inches. ( I just really wanted this rounded look for my kitchen and want to put brass trim on it to match the brass hardware in my kitchen.

I could just get the 300cfm and be done with it. My neighbor who builds custom kitchens and has installed lots of rangehoods, thinks I will be okay with the 300, but VAH doesn't think so.

Since my current hood captures nothing, I might be happy with 300. I just thought that maybe if I buy the 600 and only use one blower for now, I will be okay and then when I have to replace my roof, I can change the ducting to 8 inches. I also have the wrong roof outlet according to VAH. It's a small round one with vents on the side. They recommend a much bigger one that looks like a little house. :-)

I will have to spend some time today researching the Kobe/Zephyr hoods to see why they can have 6 inch duct with 600cfm.

Who knew it would be so much trouble to just change a rangehood.
VAH doesn't offer a transition piece, so I have to find out if I can get one at a supply place.

The reason I know what I know about this topic is that I had to do a giant "science project" learning exercise back in 2007 before I could order anything for my kitchen reno.

You have an aperture that is not larger than 3 ft by 1.75 ft or about 5 square feet (roughly). In my view, the actual flow rate on maximum should be around 90 cfm per square foot, which gets us to around 450 cfm actual flow rate. A VaH (at best) meets its claimed flow rate when the hood is hanging in the air, and there is no other restriction (pressure loss). However, in real life, the hood has exit ducting, a roof cap, and the make-up air has some restriction. (Even an open window with a screen can have an effect.)

For baffled hoods, I use a factor of 1.5 for zero static pressure loss flow rate to achieve the desired flow rate with restriction. (This assumes a reasonable hood cap or roof mounted blower housing.) While the VaH doesn't have baffles, the data that appeared here a few months ago indicated that their flow reduction with pressure loss was greater than typical Broan blowers that I have data for, so I think I'd stick with the 1.5 factor.

The product of 1.5 and 450 is 675 cfm, so I think 600 is a reasonable design point for you.

What you may find running only 300 cfm is that you can cook smelly things on simmer or reheat precooked sausages at a low power level and capture and contain adequately. Higher temperature cooking associated with griddle use, searing, or wokking will produce faster rising effluents that may not be well captured and contained.

Note: I use 10-inch duct with a 1500 cfm roof blower probably pulling an actual 900 cfm. Your 7-inch duct has only half of the sectional area, but only needs to move about half the actual flow. There will be somewhat more duct friction per sectional area with the 7-inch duct, but I would agree with VaH that the roof cap needs to be suitable for the flow rate.

600 cfm hoods with a minimum of a 7-inch duct recommendation likely can do so by using a blower that has a more vertical fan curve between the zero static pressure (rated) flow rate and the flow rate they are expected to actually operate at.

Custom kitchens seen in magazines often have no ventilation at all. After all, they only need to look nice. Food is catered, or else the staff cook in a more industrial kitchen off limits to guests.

:)

kas

Ah, I should just quit cooking and eat out. That way I can have "the look" I want. :-)
So are you suggesting I give up on this one and get a different hood that only requires 6 inch ducts? Keep in mind, I have been cooking on this new range top for 3 years now including the occasional griddle use for a steaks with no real ventilation. I turn on the one I have, but it doesn't do any good. A few times my smoke alarm has gone off!
My neighbor, ever the penny pincher, told me to keep the current rangehood to save money. Boy was he wrong!
Here is where it's going.....the old one you can see is flush with cabinets when not in use. It was the builder grade installed for use with a 25 year old 4 burner range. I have 5 now, but I will never have all of them going at the same time. Not THAT into cooking. Kitchen is 10'x10' and if you do the math for the square footage it will come up with 300cfms but that isn't taking the cook top into account.

I'm not going to proclaim that X is better than Y because there are too many factors that need to be considered, including noise, cost, and aesthetics. I was only remarking that higher pressure loss calls for an appropriate fan to deal with it. In your case, considering the eventual intent to modify the ducting, the higher pressure loss is nominally temporary. There is also the question of the money value of your time, which is a factor in how much you might want to delve into this subject and check out the myriad hoods and blowers that are potential candidates.

kas

Ok thanks, Kas. I appreciate your help! I have looked at Kobe and Zephyr as alternates. Both have 6 inch venting. I still lean toward getting what I want and eventually upgrading the venting system. I asked my neighbor how soon I will need to replace my roof. He said: "when it starts leaking." :-)
My house was built to self-destruct at the 20 year mark more or less. I've already had to re-pipe even though I had copper piping because it was too thin and I was getting regular pinhole leaks. All toilets have had to be replaced because of cracks in tanks. And now the double-paned windows are having seal failures so I have moisture in between the panes. And the list goes on.........the joys of home ownership!! The kitchen remodel started with the plastic drawer guides (or whatever they are called) breaking and the garbage disposal going belly up.

The only parts original in the original part of my house are the foundation and basement floor, furnace chimney, framing (in some cases removed, in other cases reinforced, but generally of good quality), a bit more than half the original sheetrock, and about 2/3 of the original cedar siding. A few interior doors and floors are still original. Only a couple of electrical cables (out of tens of thousands of feet) are original, and the only original plumbing is some of the exiting DWV and the town part of the water feed to the meter. Overall, the house is more than 2X the original size. The original fuse box (4 Edison fuses and two power blocks) has been replaced by four large panel cabinets and an auxiliary load shedding panel for the generator. Oak Island Nova Scotia may have a money pit, but so does NH.

kas

That's what I call my house now "the money pit."