Doing a little head scratching over the price of twin wall flue, I would like to install a woodburner in my half buried basement the quotes so far for a do it you self installation are in the region of 1300 euro thats for a clip together system with 4 bends and cowl 8 meters tall.

There is a system here in France of chimney bricks cast in clay that cost peanuts relative to stainless twin wall.

Is it a bad idea to build with these blocks up to the eaves insert flexi pipe and then just two lengths of twin wall to finnish of does that sound doable ? it would need to be sealed against rain ingress obviously.

Any one any experience ?

Yes flexible flue liner is only allowed within a masonary chimney.

I think you’d find doing it all in twin wall would be much much quicker and easier – probably the same cost also [by the time you’ve bought the blocks and fitted them, then bought the flexi and the necessary adapter to attach to the twin wall].
Exposed twin wall ‘can’ look nice.