Tom G wrote:
"a" wrote in message news0Dyj.62934$C61.53035@edtnps89...
Replaced the filter after doing some renos and noticed that the exhaust
(flue?) that runs through the cold air return portion (and thus heating
it) is just regular round pipe. It occurs to me that it would be *much*
more efficient if there were thin metal vanes epoxied/welded to the
outside and thus giving a much larger surface area for the exchange of
heat.

Are there such vanes? Can't I just make them myself?

a
Can't imagine why the exhaust flue would run through the cold air return.
If the flue were to become too much cooled by the cold air return, the
exhaust gases would not flow up and out the chimney. Furthermore, a flue
usually being somewhat thin galvanized as opposed to the cast metal of the
heat exchanger; if it were to rust through, you would be exposed to carbon
monoxide leaking into your heating system. Sounds like a bad design or
installation.

Tom G.



OK the "heat exchanger" is a 6 or 8in diameter smooth round pipe - it maybe
cast or thick gauge steel. This leads from the burner area through the cold
air return and out the flue.

In any oil FHA furnace there has to be hot exhaust gases heating the cold
air return - separated by some substance that is a good conductor. Any
would be in danger of a hole and CO leakage - that's what inspections and
CO monitors are for. What if there's a hole in the "heat exchanger" - what
runs through that - pink marshmallow?

It's basic furnace 101 - the cold return air gets heated by the hot gases
created during combustion. How else would the cold return air get heated?

a