Oil fired forced air furnace

There was hard green mineral built-up below where the oil fired furnace flue enters the the chimney in the basement. Any help would be appreciated. I suspect it is degrading mortar and sulfur.

Something is or was leaking water. Rain cap? Chimney size? Oil tech needs to investigate/verify proper installation/function and have the puke cleaned up.

Interesting. You are one of the most knowledgeable inspectors around and I would say that your instinct is probably close to the mark. The photo looks like a negative or insufficient slope on the flue
and then the foil tape looks suspect, which could contribute to your observation.

I was stumped, truly. The greenish heu through me completely off the mark.
After Brain fart dissipation, and things looking clearer, Plain Efflorescence.

Efflorescence accumulates on a masonry structure as a result of moisture passing through the chimney, dissolving the soluble salts that are naturally within the brickwork and mortar. The salty liquid then evaporates when exposed to the atmosphere and leaves the salt residue on the exterior of the chimney.

Likely condensation within the chimney structure due to flue cool down conditions and the flue not being sealed.

Flue/chimney is too cold. That is tested during annual maintenance (that probably has not been done recently).

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Just a couple of observations. The tape is fresh and I see some possible rust right at the wall. Maybe attaching new to old. Also, I lot of discoloration (moisture?) All around and above.

I agree with David.

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