I have been pursuing this issue for quite some time. My oil fired boiler is running fine as verified several times by various technicians using their efficiency and tune-up equipment. My chimney is all stainless and is fine.

When the clean out door at the bottom of the boiler is opened a red/rust color debris can be seen accumulating. This apparently is not unheard of but not so common. This appears to be the source of the problem.

Around the smoke pipe that is on top of the boiler that connects to the chimney, small particles of the red debris are found. More importantly when the furnace fires the draft pushes these pariticles up the chimney and onto the roof where the dew and rain cause it to stain my chimney and roof.

So what can prevent the red particles from forming or how can they be stopped from going up and out the chimney?