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gas & oil in same flue

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That flue is likely oversized and has been causing flue gases to condense on its sides for decades. Being acidic, those gases trash the clay liner. Of course, the gas burns at a lower temperature so those gases will cool sooner. Also, if it's a gas appliance with a 24/7 pilot those gases from the pilot will be condensing all the time.

It happens with both oil and gas flues so I wouldn't peg it to just mixing the two so much as I would to an oversized flue.



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Yes. Mixing oil and gas fired appliances in the same clay-lined flue causes deterioration much sooner than same-fuel equipment. Also, changing from oil to gas equipment and utilizing the same flue results in rapid deterioration of the clay liner as well. The condensate from the gas appliance, combined with the sulfur deposits from oil-fired equipment results in rather aggressive sulfuric acid.

I always encourage relining with both the above. It'll need it sooner rather than later. As Mike stated, the flue dimension is often oversized for the updated equipment anyway.

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