No your noise level will greatly increase and you will get no performance. It's like pinching a straw air will not pass. What are you cooking on, you might be better off dropping down to 800 cfm kibe hood which can be used with 6" round duct.

I am not sure I understand the prior response. As I figure it, 800 CFM is 800 CFM whether it is being moved by a turned down Prestige or by a Kobe. And the duct noise of 800 CFM going through 6" duct is going to be the same whether it is pushed by a turned down Prestige of by a Kobe.

That said I would still vote for something like a Kobe. In the times that you need as much ventilation as you can get, how could you possibly judge how high you can turn the Prestige without overloading and risking burning out the motor? Better to crank something all the way that is rated for your ductwork.

If you are really convinced you want a Prestige, they I say buy a cheaper Kobe for under a grand and sell it used when you do the roof and buy a Prestige then. The money you will lose will be a drop in the bucket compared to the cost of a new roof and the differential of the Prestige price. And who knows, by then you might find you are happy with the Kobe and will save yourself a few grand by keeping it.

Or I could get the 600 CFM Prestige blower and then upgrade just the blower when the roof is replaced. It's a 36" 6 burner Capital range BTW.

The more I think about it, my question would be "exactly how hard is hard?" If the GC does not feel comfortable with it what about asking a real roofer? While I am not familiar with your roof, I have a hard time thinking that installing a roof cap on ANY roof would cost more than buying a different unit, only to remove and replace down the road.

That's what I asked him. These are the reasons he gave me:

1. There are 2 roofs on there, the original shake tiles, and on top of that the metal shingles which come in sheets, not individual tiles. This is difficult by itself.

2. Since the metal shingle company is out of business, there is nothing to replace it with that would come even close to matching it.

3. The liability that may result if for some reason the roofer does it incorrectly and it starts leaking.

That section of the roof will be vaulted underneath, so if for some reason it does end up leaking, it would be very, very difficult to replace the surrounding areas underneath.

I told him I don't care if it doesn't match, but he felt very uncomfortable doing it, probably for the liability and leakage issues I'm guessing.

We have the same 1200 Prestige with a 10" exhaust. We went up to the ceiling & then a 90 bend out the wall. Local code is ok for us because we are 30" away from the window. Also some codes don't allow going form a 10 to a 6.

I wish I could go through the wall, but due to the slope of the roof that's not possible in my case unfortunately. I'll have to check about going from a 10" to a 6", thanks for the info.

I don't what's so difficult with making the hole in the roof bigger? A sawzall should do it.

It's not making the hole bigger that is yoyoma's issue. It's keeping the rain from coming through once a bigger hole is made. I think he's absolutely correct to be concerned.

If it were me, I'd do the 1200 or the 600 Prestige now. One question is whether you are interested in doing an external roof mounted blower when you replace the roof (which would be my suggestion, as it's much superior to an internal blower from noise standpoint). If that were the case I'd do the less expensive 600 cfm unit now, particularly if the Prestige would subsequently allow you to remove the internal blower and go with an external blower at a later date.

If you're going to stick wtih an internal blower now and in the future, then I would consider using the 1200 cfm unit and just running it at less than max speed. Any blower works against some load anyway, depending on the length and size of duct, number of turns, etc. A high quality blower should be able to work against load for an awfully long time without burning out. It would be easy to put it on a rheostat and run it at lower speed until you upgrade the ducting.

So, personally, I'd get a Prestige with either the 600 or 1200, run it hard, and then replace the blower with an external roof mounted blower when you replace your roof. FWIW.

clinresga, that makes sense to me, thanks. Do you know if it's possible to have an external roof mounted blower even if it won't be accessible from below (due to vaulted ceiling)?

Are you saying that by having a rheostat it limits the maximum the venting will go so it won't blow out? Thanks.

Burning a motor out seems unlikely; the less air moved the less work the motor performs.

If the transition from 10 inches to 6 inches occurs where the 6-inch section is very short, the losses may not be as bad as if a long stretch of 6-inch duct followed. Every transition in a duct system, including that of a giant hood mating to a 10-inch duct, causes loss. The 6-inch section is surely undesirable, but may be tolerable as a temporary expedient.

As for the larger section, one wants the air velocity inside the duct to be in the 1000 to 2000 ft/min range. The reason is to keep grease vapor from condensing on the ducting. If the 6-inch section causes a drop in flow such that in combination with the hood and fan the 10-inch duct flow speed is too low, a smaller duct is called for.

You might guess an actual flow rate based on pressure drop across the fan due to transitions and restrictions, say 800 cfm (possibly optimistic). Then divide that value by the duct area in square feet to see what the exhaust speed will be. In this example, one would want about a half square foot. 10-inch circular duct would fall into this range.

However, if all that can be squeezed through the ducting is, say, 400 cfm, then one would want around one-quarter of a square foot of duct area. An 8-inch circular duct would be ok in this case.

Noise effects will be motivational for roof replacement. :)

kas

"the less air moved the less work the motor performs"
the above statement is true up to a point- some motors rely on the airflow for their own cooling. Remove that entirely, and they could eventually fry.
Casey

Hmm, I don't quite get what I need to calculate kas, but it sounds like that's what I need to do. I'm going to call Prestige next week and see what they recommend for my situation. Thanks.

As usual kas has the last word on ventilation.

External roof mounted blower should be mountable without access from below, though obviously you are able to access ductwork (not clear to me where the duct is running through your vaulted ceiling). Along with the ducting, you'd need to be able to run electricity, of course, but if you can run duct, that should be no problem.

Here's the specific information on the Hy-Ex blower that is generally rated tops (up to 1400 cfm and a remarkable 60 db sound rating):

Here is a link that might be useful: Hy-Ex ventilator installation instructions

Thanks Clinresga, I guess I can use that Hy-Ex blower with a Prestige hood.