Cause:
Dust and dirt in the air had - over 10 years - been carbonised and clogged the heat exchanger fins. This had caused very high temperatures in the burner, causing the ignition electrode to deform. As a result, ignition was retarded causing a backfire effect. Boiler engineer purchased seal set, opened heat exchanger and cleaned each very narrow fin using a hacksaw blade, brush and vacuum cleaner, before fitting a new ignition electrode. This fixed the problem.
mkpaul
January 2014
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