So I have a redundant chimney I’m looking to close out, so far so normal. Except in this case someone may have been a tad hammer happy and smashed through the poor excuse for a hearth and so opened up a possible route to the vented crawl space under the floor.

So my question is – is there any good reason why I can’t ventilate the chimney from the under floor space rather than the room that I’m trying to insulate? I was thinking of some of the rectangular section ducting behind the shelves/whatever we are sticking in the old fireplace up to the chimney itself that can have a bit of rockwool flung about it to keep the draughts out.

And remove the old metal flue and get it swept, obviously.

Seems ok to me provided the floor void is vented to outside via airbricks and you vent the top of the chimney with a rainproof cowl.

Venting through the room will keep air moving in the house and thereby ventilating both house and flue which would be my preference for the health of the house, in the holistic sense.

If it were me, I’d remove the liner, flaunch the top of the chimney and insert a couple of air bricks on the side, a course from the top.
Venting as you describe at the bottom won’t present any issues as far as I can tell. You don’t need to sweep the chimney as it was previously used for a gas appliance and would have been swept before that was fitted. Even without a vented suspended floor, there should be enough adventitious air to keep air moving through. (Edit: I can’t imagine the suspended floor would not be vented…..)
Check the rest of it isn’t degraded inspecting the pointing for any signs of decomposition because this is the perfect opportunity to ensure you don’t have to go up there again for a very long time.

What? No woodburner? Best hand over your login as you leave. 😉

You could get a chimney pot replacement that’s sealed, but with little vents in it. Keeps the rain out and retains the look.

Lol I can assure you I won’t be going anywhere near the roof under any circumstances!

You say it should have been swept but the crap coming down says otherwise, its ex LA so was probably done on bonus as per everything else, for the cost of getting someone to pull the flue out I may as well get them to check.

Slack Alice, we do intend on keeping the house ventilated but have plenty of trickle vents that don’t suck all the heat out and the seagulls tend not to tap dance on them either!

Thanks though, glad I was on the right track.