Take two single-walled
flue pipes of different enough diameters and place one inside the other

that's a co-axial pipe. A dozen commercial products use it. Flue
gases flow out, exchanging heat with combustion air.

Nick sounded as if (sometimes he posts without proofreading, I can
show you some posts where he confuses the dimensions of thermal
conductivity and thermal conductance) he was proposing a THIRD airflow
that would bring vent air into the living space.... a pipe inside a
pipe, inside a third pipe.

And yes, George, it's not a "new" idea. He didn't claim it was. By the
way, what's YOUR track record vis-a-vis optimizing thermal designs? If
you post half the tutorial material I've learned from Nick, I'll sing
your praises too.