All,
My home has a high efficiency forced air furnace, with combusion air
and exhaust through two PVC pipes (about 15' runs).

While at the heated pool in our neighborhood, I noted that the pool
heater was ducted through what appeared to be a single pipe (pipe in a
pipe), with the exhaust through the inner pipe and the intake through
the outer pipe.

If I understood the arrangement, I could see a few advantages:

1) The inner exhaust pipe would have counterflow to the intake, and
should heat the intake air somewhat.
2) The inner pipe had a screen over it, to prevent entrance of
critters. I put 1/2" hardware cloth over my intake pipe to prevent
the entrance of barn swallows, but it capped over with snow in the
winter. This arrangement looked to be a little more robust.

Is this sort of arrangement available for a forced air furnace? What
is this called?

Thanks,
Chris