what causes a rusty chimney? as in rust stains weeping down the outside of the masonry and down the nearby shingles. maybe previous owner had a cheap cap on there for some time? it's curious, it's only around the one flu, but down all 3 of its external sides.
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I would ask them how they recommend getting triple wall through their roof system and whether it needs to be done now or if it is not such a big deal to do it later.