Modifying a cooker/hob extractor

Nah usually they're only used on recirculating setups to attempt to absorb "smells".

My cooker sits in what was once a fireplace and there is insufficient height for a conventional "chimney-style" extractor. I've picked-up an ex-display extractor and dismantled it, with the aim of fixing the S/S rectangular part (with filters) over the cooker and remote-mounting the centrifugal fan unit. The question is whether to put the fan in a box on the outside wall (so it's sucking the 800mm from the filter box) or to make some sort of "power bulge" in the 5" ducting (nearer to the filter box) and blowing the rest of the way. Before I do some experiments, does anyone know how well centrifugal fans "suck" compared to blowing? I have a vague feeling that they suffer from blade stall with even slightly reduced pressure, but I can't find any useful info.

It will be fine

Make sure the ductwork can be cleaned out.

the difference is trivial

I suppose it may depend on how you cook, but the danger without a filter is that the duct, the fan and the outside wall below the outlet will get covered in thick brown rancid fat.

No filter on mine. Yes, the inner shutter does get pretty grubby and so does the outer grill but trivial to take them off and put them in the dishwasher.

Nothing ever ends up on the wall below the outside grill.

It's designed to have aluminium mesh filters and would look bad without them. In my last house the filters looked and felt "manky" after a while so we put them in the dishwasher every couple of months.

Oh mine does have its aluminium mesh screen in place and that's easy to clean. I thought a "filter" would be some kind of disposable activated charcoal cartrige.

Mine has a flap that can be moved during installation to give recirculation or to exhaust through the wall. The filter consists of two layers on top of each other - the first filters fat to stop the interior (and the ducting, if any) from becoming coated and the second removes odours. If exhausting through the wall, only the first layer is required.

SteveW