Old FHA system...

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
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a wrote:

You could shove in a flue from an old water heater, if it isn't too small...
nate
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Its not code or safe to have a flue run through a return
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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.
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Tom G wrote:

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
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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.
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David L. Martel wrote:

This is my setup:
http://oee.nrcan.gc.ca/publications/infosource/pub/home/Gif/heatingOil_fig01_e.GIF
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.
a
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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.
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snipped-for-privacy@optonline.net wrote:

Good point. It is old - due to be replaced in the next year or so. Probably not worth the hassle.
a
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