She had someone to come into to look at it when I suggested doing so, having noticed that soot was falling out of the front. He said that it repeatedly failed tests on flue flow and that flames were coming out of the front...
The question that arises in my mind is why the previous engineer was unable to clean it or identify what had caused the boiler to suddenly soot up after so many years of normal operation.
Many thanks to those of you who contributed. As you implied, what the boiler needed was a good clean, performed by a competent and scrupulous person. That has now been done. There is still a slight tendency for soot to form, and there is carbon monoxide (at most 10ppm). I plan to increase the ventilation to the room for that. (The existing vent seems to be properly sized and clean but is rather tortuously routed.)
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