I often tell my clients at the conclusion of a home inspection, take care of the little things when you see them. Often what you see may be bigger than you think. The picture below shows what appears to be water stains on the exhaust of a furnace and water heater.
If that were the end of it i’d say find the cause and correct it. Unfortunately when I inspected the same flue in the attic, the situation was much worse.
In the attic we found a flue that has corroded all the way through. So now we are looking at replacing a flue that runs from the basement up three levels to the roof, a much bigger expense. And judging by the duct tape, someone knew about it.