venting a new cooker hood extractor via an existing chimney

[Rockabill]

I have created a recess (1m wide x 1.8m high x 380mm deep) in an existing chimney breast for a cooking range. This opening has a new soffit with a cut-out for a flush fitting extractor unit 750mm above the range, and above the soffit the chimney flue extends through to chimney pot level, some 8 metres above.
I can either line the flue with approx 8m of 125mm dia flexible stainless steel flue-liner to connect to the extractor (and with a condensation trap fitted as recommended) OR do I simply fit the extractor and vent it into the existing unlined brickwork flue. A separate louvred aluminium vent could then be fitted in the soffit next to the extractor to permanently ventilate the flue to nullify any condensation (but would allow some dust or fine debris to find its way from the flue onto cooking surface below).

The first choice is a lot more expensive as scaffolding and a roofer would be required (plus materials -flue-liner, cowl etc.) whereas little expense will be incurred if the latter solution is adopted. I am still concerned that venting steam and warm air directly into an open flue could lead to a condensation problem.
Your comments or experience of a similar installation will be welcome.

 

[OwainDIYer]

Venting steam and warm greasy wet air into the flue will lead to condensation and possibly other problems (cockroaches etc). I would suggest a smooth-walled duct which will accumulate less much. 4" soil pipe is fairly usual. Intumescent (fire-proof) collars where it passes through ceilings.

Also many cooker hoods will have difficulty coping with an 8 m duct.