On Mon, 16 Apr 2007 13:05:29 +0100, Ben Blaukopf
wrote:

Lurch wrote:
On Mon, 16 Apr 2007 06:57:14 +0100, Dave Fawthrop
mused:

On Sun, 15 Apr 2007 22:43:53 +0100, Ben Blaukopf
wrote:

|!Lots of questions tonight...
|!
|!The old cooker hood in my kitchen has a 4" hole at the right height, and
|! about as far left as I can possibly have the new hole if I'm having a
|!chimney hood.
|!
|!Modern cooker hoods appears to want 5", or more usually, 6" holes.

Get a 5 inch to 4 inch adaptor, that is what our local Builders merchant
offered me.

If the extractor wants a 5" hole then reducing it to 4" will impair
the performance. I did the same thing on one job and I had complaints
the cooker hood wasn't too great. Whether that was because it was just
crap (doubtful as it was some huge AEG thing that was as big as the
range cooker) or because of the vent diameter I don't know.

Presumably if it wants a 5" hole, sticking a 4" in will reduce the flow
by at least 36%. In practice, the flow reduction will be greater,
because flow speed varies according to distance from the edge of the
pipe. Possiby not much greater though - I've no particular inclination
to work it out.

It's pretty academic, because the vast majority of hoods want a 6" hole.

I hired a 6 inch core driller but ended up stich drilling the
circumference because the core was taking ages and almost killing me
up that ladder. My SDS drill took a couple of minutes and the
untidyness is hidden by the grill thingy.