This is the question of the day.

stretch

What do you think about taking the flue pipe through the plenum (80%) for the extra heat off the pipe. What would you guys tell the customer????

Steamhead

I wouldn't do that

it will cause the flue gases to condense in the flue pipe and chimney. This can cause many problems. 

Paul Fredricks_3

Well

I don't think you would loose that much heat from the flue, unless things were marginal to begin with. And I don't think there would be any real gain to the outlet air temp. But I do know that if there was a problem with that pipe the possibility exists to put exhaust gasses into the house, and that would be very bad. Don't do it!

bob_46

flue

Years ago on some basement model furnaces the manufacturers used to run the flue through the blower compartment. Sometimes they even made attempts to increase the surface area of the flue. Not a good practice. Lots of failures and the flue gasses go right into the air stream.

Paul Pollets

Bad Idea

Illegal and forbidden by all mechanical codes!!

Paul Fredricks_3

Of course

Low boys with rear flue are set up that way, but they are a fairly substantial piece of metal. Then again, I've seen plenty of Carrier low boys/rear flue that are anything but substantial and are rotted away.