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?
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
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?
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
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?
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
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.
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