A little help from the brain trust please

[mgbinspect]

The photos below are of an exhaust system for a 1966 oil-fired furnace

Of course I wrote it up as a suspected oil-burner adjustment problem and called for the works.

I'm curious what is causing this peculiar type of residue and leaching. I know that H20 is a big biproduct of natural gas exhaust, but I wasn't as sure about fuel oil. Could it be a restricted flue?

Thought maybe someone will recognize this specific condition.

Thanks in advance.

Mike

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[randynavarro]

I see that frequently, although more so on gas units than oil-fired.

It's the corrosive particulate from the metal flue oozing down and out of the seams 'cuz the exhaust is condensing too quickly.

If you do a bit of searching on this site, there have been at least a few discussions.

[Bill Kibbel]

I know that H20 is a big biproduct of natural gas exhaust, but I wasn't as sure about fuel oil.

H20 is a byproduct of combustion. You ever notice water dripping out of vehicle exhaust pipes?

You probably don't see it there as often as with gas because oil burners operate at a lower combustion efficiency. I see it quite often as I inspect a lot of buildings that have been converted to oil heat when the chimney was designed and sized for coal. Coal heat kept chimneys warmer and didn't have rapid on/off cycles. With oil, these chimney flues are now at a lower temp., resulting in significant (and very acidic) condensation.

Was the flue oversized? unlined? Chimney on the exterior of the building?

[mgbinspect]

It's 8" diameter with a few feet of horizontal before hooking up with a masonry chimney (9 X 13) so I suppose it cools down darn fast. It was just odd for an oil-burner around here - especially at every joint and hole. And, yes, Bill, the chimney is on the rear exterior wall - the NW wall, at that - never sees the sun.