K
kirkgas
Im pretty sure its from the center of the flue to the start of the opening i know not much difference and a little padantic but can sometimes make a difference that 35mm
c'mon ryan think about it, it can NEVER be the centre of a flue it must always be the nearest point, eg you have a 100mm flue and measure from the centre, then go to another house that has a 150mm flue and again if you measure from the centre there is more flue within range of the window, however if you measure every flue from its nearest point then the diameter of the flue is irrelevant and therefore the volume of POC's getting to the window will always be the same