Under some conditions, adding a roof blower to a system that has a hood blower would significantly increase flow. Control would be the big issue, and requires that both fans use the same type of control, and that the control used be able (or be swapped with one able) to handle twice the current. If you are getting a tailor-made (bespoke) hood, you could order a suitable control for the initial configuration. Both fans need convex fan curves to avoid any hunting between fans. The pressure losses through the baffles and restricted duct, along with any MUA restriction, would determine the actual flow rate. kas

In my building every 4 individual apartment ducts are combined into a common duct. Oversized external blowers take the "mass" from these big common ducts. I just need to get air over to that common duct; but i have to go through a 6" duct. The 6" duct cannot be increased in size. Mine runs about 4 feet until it dumps its air in the bigger duct. Three years ago we put in our bathroom a remote inline ILF fan in an ILFK kit. Nutone Broan. Love it. It's such a nice little air noise pulling a lot of air. Like taking a deep breath very slowly. Love the little button shaped hole in the ceiling that we can close when we want to. http://www.nutone.com/product-detail.asp?ProductID=10476 I called them and asked for a Variable Speed Control for it. Then I asked about the HLB3 in the kitchen and a Variable Speed Control for it too. Got that too: VS64 rated for up to 5 Amps; more is better; I could also get a 3 Amp one. The outstanding question is whether silent/quiet operation is still going to be in the cards given that the output vent diameter would be reduced from 8" to 6" over a few feet length. I'm not sure how to determine the increase in noise and the reduction in strength that will come from installing an 8" inline blower on a 6" duct. Later I'll get part numbers for filters and cover grills to suit SO's preference, if we go with this blower. David

I am not a HVAC guy but I did run ducts prior to drywall in a basement finishing project. I was just tying into another system but I wanted it to handle the entire 4 tons if I zoned the system. That would only be for heat and it is an Infinity system so it would cut back the flow if the ductwork was inadequate - this makes a huge difference compared to what you are doing. I did a lot of research and in the end, I overdid the ductwork to be safe. Yes you can go too big but it is harder and the distances were short and the loads are low. The ducts are not oversized for the possible max airflow but they are oversized for the load. By memory, I had 2 8 inch runs that were 10 feet and 3 9 inch runs that were 20 feet (roughly). I haven't zoned it yet and it works fine. So it really is just stealing a ton or so off the main system. The one thing about flow is that a rating does not guarantee the flow. Obviously a 25 ft run of 8 inch flex duct is not going to flow 160 CFMs as easily as a 5 ft run will. The longer runs need to be sized bigger to compensate - I believe that is what a ductulator does for you.

The way I think about it, a 7x7" duct is 49"^2, whereas a 8" circular duct, which seems standard, is around 50"^2, so it's quite similar. The actual math is a lot more complicated than that. It generally requires solving partial differential equations which is not something I am prepared to spend my time on. But assuming you actually have a full 7x7" and smooth wall surfaces, there is an approximation that should allow you to estimate the equivalent dimension of a round duct: Deq = 1.3*(x*y)^0.625 / (x+y)^0.25 = 1.3*49^0.625 / 14^0.25 = 7.652 If we assume laminar flow (probably not quite true, but a fair approximation as long as the walls are smooth), then the Poiseuille equation applies. It states that the flow rate goes down with the fourth power of the radius. For a 7x7" duct, that would be about 16% less than a round 8" duct or about 65% less than a round 10" duct. Additionally, the flow rate goes down linear with the length of the duct. Let's say your 40' duct is about 5 times longer than typical residential ducts. Let's also assume that hood manufacturers would normally recommend a 10" round duct for the type of hood that you have. Then you only have about 7% of the capacity that the manufacturer recommends. The biggest problem is obviously the length of the run. But the dimensions are also a very real factor. And the smoothness of the walls (if the duct isn't lined with metal but maybe just brick) shouldn't be neglected either. In an ideal scenario, you'd either find a different location for where to vent to the outside resulting in a significantly shorter duct, or if that isn't an option, you'd increase the diameter of the duct dramatically. Something like 12x12" wouldn't be a bad idea. It's probably OK if you have to reduce back to 7x7" for the last two feet on the roof.