The flue on my 15 year old glowworm complheat boiler (Concentric fan flue) is corroding, due to condensation in about the outer two thirds of its length. The corrosion was blocking about 25% of the flue area, and causing staining on the wall below the flue.
The flue is installed absolutely horizontal, resulting in the condensation sitting in the pipe. It is *not* a condensing boiler. Return temperature (apart from in initial warm up) is high enough to prevent condensation in the boiler.
Would it be worth adjusting the eight of the hole in the wall, to permit the flue to drain? If so, into or away from the boiler. I suspect the latter would be preferable to the current situation, but wonder if it would increase tha staining. The outlet is in a fairly sheltered (from rain) alleyway, so the staining doesn't get washed away, and is only removable with acid.