6” not sufficient. It needs to be 8” or 10” for good exhaust. What hood is this? Please say the brand name and model number.

Flexible duct for a hood exhaust is against building code everywhere, so if that is what this hood came with, there is something amiss with the hood. Pictures would help since I could not follow what you meant by an ”open hood”. I also could not follow what you meant by ”have some part of the pipe exposed under the ceiling...My question is - how much should the 6 inch galvanized pipe be exposed from the ceiling? what is the code?“

@M Miller - thank you.

I put a picture to explain what my question is - it is marked in red question marks. How many inches should the solid pipe hang below the ceiling? I will attach the flexible duct that came with the range hood to this solid metal 6 inch pipe.

By "Open range hood" I meant - range hood that's not covered by upper cabinets. (As in the picture above). Any references could be great!

Thanks for helping :)