In addition every manufacturer has a different spec duct size based on the rpm the motors spins and how

much ait they sre trying to push. Reducing the size and making hard turns in the pupe cuts the efficiency. follow the mfg guidlines for duct size

"so even if its barely functional"

Under this rule anything is suitable.

so even if its barely functional due to the ducting being in the cabinet

What does that mean? The ducting can be fine and functional whether it is hidden in cabinet or not. Instead, the key factors are (A) is the hood ducted to the outside, and (B) if so, what is the duct diameter.

In addition every manufacturer has a different spec duct size based on the rpm the motors spins and how

The vast majority of good-quality hoods specify an 8" diameter, or when the cfms of the hood are quite high, a 10" diameter duct. It's not like there is a broad spectrum of different duct sizes by different manufacturers. You can find hoods needing a 6" or 7" diameter duct, but those are typically for hoods with lower cfms, and cater to the many homeowners who have preinstalled ductwork that they cannot make larger.

Go with 8" or 10" diameter and make sure the insides of the duct are smooth (i.e. no flexible tubing). If possible, avoid bends and avoid having to use rectangular ducts. But sometimes, that's all you have to deal with.

For a 36" gas powered range, you probably want to look at about 1000CFM. You frequently see a guideline here, that optimal flowrate through the hood aperture is 90 feet/min. For a 7 sqft aperture, you would nominally reach that at just over 600CFM. But in practice, you hit all sorts of performance losses. A frequently quoted rule of thumb is to add another 50% to account for that. And that gets you to almost precisely 1000CFM.

There is no reason why you would need to buy the hood from the same company as the range. A custom-built hood is great. And in fact, some of the commercial designs might work better for you than an off-the-shelf consumer hood.

If you are worried about hitting your head on a 27" hood, install it higher. If you think that this would impact efficiency too much, install shrouds on the side (upper cabinets can sometimes do double-duty as shrouds).

A big part in how well a hood works is make-up-air. If the hood can't suck in air, then no matter how great its design, it simply won't be able to do its job. In some parts of the country, all you need is a permanently open window on the other side of the room. In other parts, an expensive heated MUA system is needed. And of course, if you have any other gas powered appliances (e.g. furnace, water heater), code might make you install a MUA system anyway, as backdraft is a very real problem. CO poisoning is no joke.

In general, if you have the option, an external blower is a much better choice than an internal blower. It'll inevitably be quieter. But if you want it even less noisy, investigate whether you can install an inline noise muffler. These things take up space, so many people simply don't have a way to put them anywhere. But they are effective.

to clarify what I meant to say was even if the cabinet over the hood is barely functional because it is filled almost entirely with a big duct, I still think I like the aesthetic of having the cabinet there.

I would use whatever diameter pipe is recommended by the blower, and whatever CFM blower is recommended for the range/hood.

I am a little surprised to hear that a remote fan is louder than an inline...is that the case when you are standing at the range? or do you mean if you stand outside next to the remote, its as loud as standing inside next to an inline? I thought the remote was loud outside, but inside it was just moving air...whereas with an inline, you have the sound of the air moving as well as the sound of the motor.

You can not get away ftom the sound of moving air, no matter what. And that is the vast majority of sound when you do either online or remote.

A wood hood will have 2-4K worth of cabinetry involved. Plus the 3-7K cost of the ventilation components. Plus the 2-12K cost of the MUA.

Yes, moving air does make sound. But that's particularly true where it can't move in a straight line. This would be at the baffles (not really much you can do about that) and when it goes through the fan. That's where placing the blower remotely would help.

How much it helps exactly does depend on a lot of site-specific factors. So, yes, if money is not a huge concern and if you have the space, I'd go for a remote blower with inline sound muffler. It's going to be better than an inline blower. But depending on hood design, flow rate, and bends in your ducting, it might or might not make as much of a difference as you would like.

Getting a custom-built hood by somebody who actually knows what they are doing, that is something I would absolutely jump on. It'll probably look more industrial, but that's a look that I am personally comfortable with anyway.

Most commercial hoods are considerably quieter than residential hoods and are much more effective. I believe this is primarily due to the larger aperture (e.g., most have a depth of 30" - 42") and to a large containment area (that residential hoods largely lack) that allow for lower air flow (velocity and relative volume).

If OP is having a hood custom made then insuring a large containment area and a large aperture opening should allow for lower airflow and improved quietness and function.

More: https://www.gardenweb.com/discussions/5161173/hood-faq