On Sun, 29 Dec 2019 23:58:54 -0800 (PST), harry
wrote:

On Sunday, 29 December 2019 23:35:46 UTC, Nick wrote:
We have an ancient (40 yrs+) New World back boiler in our kitchen
fireplace. Serviced annually and still working well. Somewhat
underpowered as the place has been extended. It is going to die sometime.
My plan is to replace it with a boiler on the far side of the wall.
Would it be practical or legal to pass the boiler flue back through the
wall and couple it to the existing lined flue which exhausts through a 7
metre chimney?
Thanks,
Nick.

Your new boiler will be condensing, another reason why you'll need a different arrangement.

and the whole flue will need to be of the coaxial type will it not, so
can't use the existing lined flue. Plus 7metres is a long way for a
boiler fan to push the exhaust.