Don't know where you got your information, but the Fantech FKD 8XL is rated 836 cfm at zero static pressure. That's your best soluion IMHO.

It's the smaller version of the 10XL wee have and I love Fantech products. We have their remote PB230 bathroom fan as well and it's also fabulous. Suprised that you had trouble reaching tech support: they have always answered first time and have been very helpful to me.

The alternative if you just had to get over 1000 cfm would be to use a transition from your 8'' duct to allow connection to a 10'' blower. This has come up before: there is certainly a disadvantage in "throttling" the blower with smaller duct, and it's conceivable that the higher continuous static load on the blower might contribute to shorter blower life, but I doubt it's really that significant a factor, especially if you don't run it flat out all the time. However, I don't really think you'll see a huge advantage over the 8XL as I think the duct size, more than blower capapcity, is the limiting factor in your performance, particularly if the duct run is at all long.

Here is a link that might be useful: Fantech FKD series specs

Thanks clinresga, I have to check if we can possibly use the inline model. It has to vent through the roof and based on the location of the ductwork we may not be able to route that way.

I have been looking at the Fantech exterior models RE8XL/REC8XL which are rated only 409 CFM at zero static pressure. Not sure why such a big difference.

I did talk to the Zephyr customer service about using a transition from 10" to 8" with their blower, but the person I spoke with said that would result in a very noisy unit unless we keep it at the lowest setting (which is again around 300 CFM) which obviously defeats the purpose.

Keep looking!! I think there are great options out there still.

If you need a roof mounted exterior blower, how about the Abbaka HyEx10 external blower? It's a very pretty, extremely low profile unit which according to the website appears to use 8'' duct and is rated at 1000 cfm. We almost went with the 1400 model until my contractor nixed it, as he was unwilling to go out onto our very steep roof to mount it.

I also find the Zephyr rep's comment to be nonsensical. If you used a blower with a 10'' duct collar, you would mount the transition ducting right at the blower. Since it's mounted on your roof, any turbulence noise generated would be on your roof, not in your kitchen. It is true that pulling a very high volume of air into an 8'' duct will create more airflow noise at the point where your ductwork enters the hood, but that will be true no matter whether you use a transition or not, for a given cfm flow rate. So, in my mind, using transition should not really worsen the situation. Maybe another of the ventilation folks out there can see another reason not to use transition but I'm not convinced.

Still, I think the best solution is to find a high flow blower that uses 8'' duct. I'd contact Abbaka.

Here is a link that might be useful: Abbaka external blower

I am looking at Abbaka and it does have an 8" version which sounds good. Their inserts are very expensive so I would have to go with something else, like a ModernAire insert then.

I also see your point regarding the Zephyr. The problem is we don't have an 8" round duct, what we have is an oval, or more exactly a rounded rectangular duct, which I thought was equivalent to an 8" round but actually it isn't. It is 11" x 3", which is about 34% less of an area than an 8" round.

So we'll have one transition from an 8" round to the rectangular duct above the cabinets (hidden by the crown molding). From there, it is possible that the rectangular duct goes all the way to the outside wall, in which case we'll have to have a transition from that right to the blower. It is also possible (I have to ask DH to check in the garage attic) that we might have another 8" round duct which connects to the rectangular one at some point before the outside wall.

Now, do you still think that the difference from an 8" to a 10" blower would be insignificant considering that we have a smaller, 11x3 piece in between, or does it make it more of an issue?

Wait: is it really 11 by THREE inches or is that a typo?

11''x3'' would be pretty small duct--roughly rectangular means about 33 sq inches, which works out to the equivalent of 6.5'' round duct. Better than your 5'', but still pretty limiting.

I am not sure I understand your physical constraints, but is there any way to fabricate somewhat larger duct? If, for example, you could get 11'' x 5'' rectangular duct, then you're at least up into the 8'' round duct range. A sheet metal or HVAC shop should be able to fabricate that at a reasonable cost given it's a short run.

Otherwise, I don't think there's any way you're going to come close to pulling 1000, or even 800 cfm through the equivalent of 6'' ductwork, and with that restricted a duct, I do agree now that air turbulence may be very loud. I still think I'd go higher on the blower, and just use a speed control to run it lower most of the time, but hooking a 1000 cfm blower to that duct will be like trying to blow out a fire through a drinking straw I fear.

I'm not sure if this will help but we have a Dacor remote blower and I'm pretty sure it has 8".

* REMP16 Remote Blower
o 1,000 CFM (994.8 CFM at 0.0" static pressure)
o 10" intake

* REMP3 Remote Blower
o 600 CFM (620.5 CFM at 0.0" static pressure)
o 8" intake

I know this is an old posting, but I'm wondering what you ended up doing and how it is working out? I have a similar (but worse!) situation where I could only fit a 6 inch duct (2 turns and 35 foot run to the roof) and am now looking for a solution (and my construction is all finished - can't re-open the walls). I just got off the phone with the Abbaka rep and am thinking of using their 1000 cfm for an 8-inch duct. Has anyone tried this or something similar? Thanks!

Kobe makes hoods (~600 CFM) that work with 6" ducts. My DCS hood (older model) is a 600 CFM hood with a 6: duct. Better to get a hood that is made for a 6: duct than to use one that requires a larger duct.