wrote:

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.

Some gas appliances and pellet stoves work that way.

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)

Interesting. I don't recall those mistakes. I archive most of my postings at
http://www.ece.villanova.edu/~nick. If you send me a list, I'll fix 'em.

he was proposing a THIRD airflow that would bring vent air into the
living space.... a pipe inside a pipe, inside a third pipe.

No... just 2 pipes, with room air vs outside air flowing back to the
stove in the space between them.

And yes, George, it's not a "new" idea. He didn't claim it was.

It may be new, but it seems obvious to someone "skilled in the art."

By the way, what's YOUR track record vis-a-vis optimizing thermal designs?

George says he's an expert. Let's try an extremely simple test. If 10 cfm
of 70 F combustion air warms to 800 F before it exits a woodstove to enter
a perfect conterflow heat exchanger and 560 cfm of 70 F room air enters
the other end, what's the temperature of the room air at the other end
of the heat exchanger?

Anyone can answer, but it would be fun to let George give it a try first.

Nick