I have done some study of this topic, but I am not technologically trained. This is what I have learned: It is best to keep your venting duct as short as possible and for it to have at most one turn, which needs to be gradual, because a sharp 90 degree turn, for example, would lower exhaust capacity/efficiency and might create other problems. Venting down is not a good idea because heat rises; take advantage of that and vent up, or at least sideways.

The cfm capacity of a system, as claimed by the manufacturer is based on certain assumptions about its installation. If not installed properly, actual cfm will be lower.

Now, for my personal experience: When we moved into our home in 1991, we had a Broan rangehood installed with a remotely located fan motor. We have a one-story house, and the venting duct goes straight up into the attic and vents to the outside through an outside wall. I have not measured the length of the vent, but would guess it's about 15-20 feet. I believe most venting ducts are rectangular in shape starting from where they are attached to the rangehood up to the point where they approach the fan motor, where they are attached to a round duct for a short distance to where they end to the outside. You will need to take into account the actual size of your venting ducts to make sure they conform to the size recommended by the manufacturer. Some systems vent through the roof. Ours vents through the outside wall in the attic. I can see it from outside; it's a round, white metal (probably aluminum) thing about 24 inches in diameter with several concentric metal circle rings. The exhaust is dumped, I believe, through the spaces between the rings.

About noise level: The remote motor installation is definitely quieter, but you do still hear the rushing of air at top speed. At medium speed it's very quiet. Some of the new rangehoods have very wide air intake openings that cover nearly the whole undersurface of the rangehood. That should cut down on the air movement noise some. Ours is old and has just two relatively small air intake vent openings, and I suspect that that contributes to the air noise.

About cfm capacity: Ours is quite low by current standards, only 235 cfm. The new Bosch gas range we bought recently recommends at least 300. We're thinking of updating, but are in no hurry. We're a retired couple and rarely cook up a storm. The cfm you will need depends on the amount and type of cooking you plan. If you're going for a 6-burner range or cooktop, it seems you'll be creating a lot of heat, smoke and splattering grease (if you do heavy frying). You'll likely need a system that can go to AT LEAST 600 cfm, but you may use that max capacity only occasionally. If you go much higher than that you'll need to take into account the negative air pressure the system will be creating in your kitchen. Simply opening a window to compensate can work in many situations. But if you live in a very cold (or very hot)climate, you may want to avoid opening a window, and prefer some kind of air intake system integrated into your exhaust system, like they have in some restaurant kitchens. It all depends on what you want and how much money you want to spend. Like everthing else in this fascinating world of home renovation, the sky's the limit.

Good luck.

Check out the attached thread link I started. It probably has more info than you need.

Keep in mind that short duct runs such as you will have could be noisier with a remote blower. Wolf cautions against this... but I don't exactly know what they mean by "short" either... could be up a couple of feet and out the wall. Going up a few feet and then six more may be OK.

I would check out the Best by Broan website and read about their remote blowers. They have lots of technical info including fan curves.

With a 6-burner AG Wolf or Bluestar you will want significant CFM capability and that will require a 10" diameter duct. The "trash can" silencer really is about that large. We have a Fantec 10" and it is about 14" in diameter and 3 feet long. It's going in the attic, just below the remote blower.

Good luck.

seattle tom's link has some great technical data and I posted to his thread early on but never followed up but my contribution would have been lost in the technical data. Now, and certainly no disrespect intended, back to the 101 version as I believe you asked.

It has been verified that turns and duct size do have an impact on effectiveness of actual delivered CFMs. However, I would say one 90 degree turn would not be detrimental. You can also pit length against size and turns. But one other thing that has been confirmed is that if the remote blower is too close to the hood, then you lose the sound advantage of going remote.

Check with others but I dont know that 6 feet will get you there. With the 90 degree turn, that may help block sound but I would not count on it a lot. These are metal ducts that carry sound well for short distances. Also, running through a floor system, it sounds like you will be challenged to get a silencer to fit.

SO, if you are in the design phase, can you find a way to build a chase in the second floor bathroom/space to accomodate a necessary duct size to the attic?

This will give you significant distance b/t the blower and hood and, with a hefty duct size and only lazy turns, the aded distance may not be an issue. At ay rate, once on the roof, you can easily oversize the blower at minimal cost.

As I mentioned in my post to seattle tom's thread, mine is a 2500 cfm remote about 15-20 feet away through a 12 inch duct. Like the poster to his thread, I have a 60 inch Blue Star that spec'd at 1,800-1,900 CFM max req'd capacity. In the system I choose, I think they had 1,800; 2,000 and 2,500 blowers. Like the poster in tom's post, I felt the 1,800 was plenty but the 2,000 was better so I choose the 2,500 b/c it was offered and only $250 more. I cannot hear the motor at all as evidenced when you turn the unit on. The only sound is the increasing volume of air being pulled through the baffles as the fan comes to full speed. A silencer will not help that. That said, my hood is incredibly quiet. Having the 2,500 CFM and an infinite speed control, I use the fan mostly at low-50% and it is almost silent. It quickly wafts away all output from cooking. On a couple of occasions so far (we have only been in our NH for 6 weeks), I have had some really hot stuff going on on the range and producing a lot of smoke and vapor. A quick flip to high and it was wisked away without any apparent effort at all. That said, the fan is still quiet enough to carry on a normal conversation over. The 900 cfm internal blower at the old house (Wolf) at the lowest setting ws difficult to have a conversation over.

As I understand it the quietest config in order of best to worst is: remote - outside; remote - inline; and internal. The most efficient best to worst is: internal; remote - external; and remote - in-line.

If you are going with a 36 Wolf or Blue Star range top, I recommend a 1,200 - 1,500 remote external blower system or a 900-1,200 internal system and ear muffs. I also recommend oversizing the hood a bit to create a capture area larger than the cook top.

HTH

one of your best guides for how much cfm you need is how much you have now and how satisfactory it is. With your new stove, once the new wears off, you might be cooking a little more than you do now but chances are not much more. if your current hood is adequate or almost adequate then I would get one a little bigger. Regardles of what your stove is rated for, you only need venting for the amount of cooking you are actualy doing. How often are you going to have 6 burners going on high at the same time?

I had a cheap builders hood for years, maybe 150 cfm at most. Seemed to be adequate most of the time, although very noisy on high. I replaced it with a 400 cfm Fantech external blower. We never run it on high, about half speed ocasionaly, and usualy on low.
The new hood is a GE Monogram under cabinet 36". The full hood area is filter. I took the internal blower out and trashed it, modified the outlet from 7" to 8" added a fantech backdraft. I made a homemade silencer because I only have about an 8' run to the outlet.
This is very quiet, and more capacity than i will ever use.

Almost any hood can be easily modified to work with an external blower. This will usualy be quieter. Also easier to service or replace if it ever breaks.

In-line blowers are best, they perform like remote blowers only they are not exposed to the weather and it eliminates having a big box on your roof or wall. They are also very quiet. Remote would be the next best and last would be internal. Problem with internal blowers is that they push air side ways before gathering. Remote and in line are like sucking through a straw, instant gratification unless you restrict the duct run, then it's like pinching the straw and reducing the flow.

I wouldn't characterize inline blowers as the best, just a different approach. This is what I didn't like about them:
- The Wolf version only came in one CFM rating that was too low for my requirements.
- It looked like it might be more work to install since the blower unit must be supported.
- It required transitions to match round duct to the rectangular inlet and outlet so it took up a lot of space.
- There wouldn't be room in the attic for a silencer unless the ducting ran horizontally.

Bottom line for me was since I needed to cut a hole in the roof for either a vent or remote blower I might as well use a remote. In our case it will be hidden from view and since our weather is mild I don't forsee any problems.

If I were running ducting horizontally and out the eves I would have seriously considered the inline. No one choice is best for everyone, it really depends on your requirements and venting configuration. The vast majority of folks use internal for ease of installation.