Ok so I know via the excell sheet that my stick burner is going to need a 36 inch exaust stack.. My question is what drives the length and diameter? I have looked but cannot seem to find a real answer. Does the length of the pipe help create back pressure and regulate flow, or does it have to do with pulling air through the system with heated air, or none of the above???? somone has got to know the reason behind it all. Also should I put a butterfly valve on the stack? everyone seems to say that you should just let the stack run full open and control flow from the fire side, but I see about every large scale stick burner out there has a valve. ???