hard to tell what's going on from here. Can you ship that chimney to me F.O.B. Mass? ;-)
It simply could be the collapse of a grossly oversized flue with the continued operation of the boiler coating everything with BPM. Since there is no wythe wall btw flues as the code requires, boiler flue gases can easily shunt over to the Fp and be drawn down into the home.
You've already confirmed this flue can longer perform its intended function and needs immediate relining. The entire chimney should have a Level II inspection. The boiler needs to be checked and tested if it is making all this BPM. Looks like oil BPM.
What's venting into this? Oil boiler alone? Gas WH? What do they look like?
The cracks in the Fp flue could have been caused during construction, seismic events, or previous
chimney fires where the flue has since been brushed out and the rain washed clean (no cap). No need to speculate on what happened. You did a good job noting there is a mjor problem that needs immediate attention. Spotting damaged flues you are going to find is one of the most universal defects in a home and one you can save buyers from. Many of the liners I do come right after settlement where the new owners eat the cost. They could claim hidden defect if they ever had that chimney serviced and were informed on a report. If no report but was serviced, there may be recourse against the sweep.
The damage should have been repaired by the homeowners. They are hoping such defects get passed on to the new buyers. More and more buyers are going to fight back and push this cost back on the original owners and any inspectors who failed to call it out. All David had to do in this case was get on the roof and peek in. Now, he has clear grounds for a recommendation to repair along with a sense of urgency as this not affects the long term durability of the home but could become a life threat or at least adversely affect the operation of the heating equipment. Inspection and testing are in order to guide the appropriate repairs.
