In message HJ_5o.20159$VW2.12704@hurricane, ARWadsworth
writes
sm_jamieson wrote:
I will be fitting a cooker hood (not done one before). I guess it will
be one with a squarish brushed metal riser. Some questions:

1. Is the fan usually integrated into the hood, or is it above the
hood in the riser, or in the ceiling ?

It will be in the hood. The metal riser is just to look nice and cover the
ducting.

2. The first stage will be to fit the ducting into the ceiling so the
ceiling can be plastered up etc. There will basically be a stub of
pipe sticking downwards out of the ceiling. When you later come to fit
the hood and riser, how much tolerance do you get left or right to
position it ? For example, assuming a flexible pipe is used for part
of the connection, you could move the duct left or right by about
inside riser width - outside duct width / 2.

Extractor hoods should use 5 inch ducting so you do have a few inches either
side to move your pipe.

I briefly considered ducting the output from the cooker hood on my
wife's flat (rented out now). The fan is mounted in the hood and outputs
vertically through a stub of duct, probably 5" x 5" as you mention.
Plenty of space along the top of the kitchen units but a concrete lintel
prohibiting access through the cavity wall.

4" duct and drop down the wall in rectangular section to an existing
vent was do-able but never happened. Oh well! See how the mould goes
this winter:-(

One obstacle was adapting the 5x5 to 4" round.

regards

--
Tim Lamb