Quote Originally Posted by Dairsie View Post
(extraction goes from large to small basically as regards ducting and should be flowed accordingly and planned by your extraction company when they deign the system.
That's not true ... if you want to get a good a vacuum as you can at each outlet you should not progressively reduce the size of the pipes. The flow resistance of a smaller pipe is greater, so the pressure difference at the end will not be as great. Think of each pipe as a resistor - doesn't make any sense to put a bigger resistor at the end.

I wouldn't use anything less than 6" diameter.