Hello, I have a typical gas furnace, 80% efficient, so it uses a
chimney. A 4" metal flue pipe comes out of the front/top of the
furnace, wraps around towards the back towards the wall behind which is
the chimney. Total of about 6 feet of 4" flue pipe, looks properly
installed. The 4" pipe goes into an 8" pipe which goes into the wall,
and the chimney is inside the wall right there.

My gas bills are horrendous I should add.

Tonight for some reason I felt the flue pipe while the furnace was
running and the thing is hot as blazes. Even at the end of the 6' run
of 4" pipe, you can't even keep your finger on the flue pipe for more
than a couple seconds cause it's so hot.

Is there some way to capture this heat ? If so, what do I look into ?
I must be heating half the country.

Thank you !