On Tue, 10 Apr 2018 12:50:48 +0100, wrote:

Our range cooker is where the old kitchen fire was, so there's a
redundant chimney (long-since blocked off) directly above it. I'm
thinking of installing a liner in the chimney and using it as an
extractor outlet. Any reasons why this might not be a "good thing"?

Depends on how much you like oily brown condensate dripping into the
food you are cooking.

Grease and oil will coat the inner surface after some time and while
some will drip back the rest will remain and in the event of a pan
fire on the hob will turn a difficult problem into a really serious
one. To minimise the risk of fire spreading up the flue you should
fit a fire flap/damper. Using a filter on the fan will not
significantly alter the amount of oil/fat going up the chimney and
most will condense on the chimney liner.