From my time doing HVAC and installing countless range hoods, I can tell you the following
- A popular standard during rough-in is ~6" down from the ceiling. This can change depending on a room's particular layout considerations, eg. cabinetry.
- You'll need at least 4" at minimum to comfortably attach your range hood with room to tape the seam. Anything less than this will lend frustration.
- Don't position your pipe too far against the wall, or you'll be making a headache for yourself there too. If you make it flush against your drywall, you're going to have a heck of a time nicely taping the seam. 1-2" is plenty of finger-space for taping/maintenance.
- Don't make your run too long - most exhaust fans will struggle to maintain pressure past 25-30' of straight duct. Remember every 90-degree bend adds the equivalent airflow loss to 5' of straight duct. 3 90-degree elbows reduces your airflow by fifteen feet!
- Do your best to ensure at least 2' of straight duct between each 90-degree elbow. Airflow is allergic to sharp u-turns.
- An ideal run will go straight up from the stove and out the roof. If there is additional building in the way of this ideal path, an adjacent exterior wall providing sufficient space to mount the wall/roof cap.
You say you're installing an open-range hood. I assume you mean by this it's free-floating in the middle of the room hanging from the ceiling, and not mounted to the wall along the side.
In this case, if the building layout supports it, you may consider running straight pipe right down from the ceiling/roof above where you want the range hood. This will eliminate the need for any airflow-reducing elbows, and should leave you with a very comfortable amount of working space to install/connect the range hood.
As a final point or two:
Refer to the product installation detail for the range hood you are installing. It should offer you manufacturer-designated guidelines which specify the diameter of duct appropriate for your exhaust fan, as well as the maximum length of duct the fan can sufficiently push air through. Remember to apply the 5' length reduction for every 90-degree elbow.
I'd strongly recommend using snaplock/straight duct over flex duct for the bulk of the run.
- Snaplock will not degrade your airflow volume like flex duct, and it won't degrade over time like plastic flex duct, requiring eventual maintenance and replacement.
- Plastic flex duct is additionally a terrible choice for a kitchen hood range due to the environment of hot oily air.
- Metal-foil flex duct may last longer, but will also wick up oils over it's lifetime, presenting a possible fire hazard.
I'd strongly recommend attempting to minimize the number of curves and bends in your run of pipe.
- Every 90 degree elbow reduces your airflow a similar amount to 5' of straight duct - this just gets worse with flex duct.
- Too much loss in airflow results in a range hood that cannot effectually extract the fumes from a stove.
- A range hood that's installed but doesn't move air is a fiscal/functional waste at best, but it may also catch the attention of a building inspector checking your work against code.
And that's pretty much that.
If you would like to know more specific nuances than that, a web-search for "range hood rough in" will bring up a plethora of websites with similar words of advice. Good luck :)
