On Mar 3, 2:16*pm, a wrote:
David L. Martel wrote:
a,

* *Don't beat up on Tom because you don't use the proper terminology, use
the proper terminology.
* *You have a cold air return. It brings cold air from the house's interior
to the heat exchanger. The heated air then goes to your house. This air flow
is driven by a fan. There's usually a filter.at the entrannce to the cold
air return.
* The heat exchanger sits inside a combustion chamber. This is where the
fire is. Hot gases are exhausted up to a chimney.
* *Now you seem to indicate that your chimney pipe ( a metal pipe) runs
through the cold air return prior to exhausting to the outside and you would
like to attach some vanes to the chimney pipe. Is this right?

Dave M.

This is my setup:http://oee.nrcan.gc.ca/publications/.../Gif/heatingOi...

The heat exchanger, I suppose *is* on the hot side - but the flue *does*
pass through the cold air return in order to impart more heat to the
incoming air.

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I've seen that setup in a very old furnace, like 1950's vintage too.
Not sure if they still make them that way in new oil fired units.
From a practical standpoint, I doubt if adding fins to that short
length of pipe is going to be worth the trouble. I don't see how you
could weld it. In the furnace I was familiar with , that area was
just about inaccessible. And I don't think you want to be welding
old and potentially thinning pipe anyway.

If you glue something on, it has to be glue that can withstand the
high temps and also has reasonable thermal conductivity. Even with
some adhesive designed for that, I'd be concerned with the glue
potentially releasing gases into the air stream.

I think the key to this is the fact that you say the furnace is old.
How old is it? If it's decades old, then the whole thing could be
relatively inefficient compared to a modern unit.