The myson apollo has an interrupter fitted to the thermocouple lead, and the interrupter is connected to an overheat stat. It sounds like the overheat stat is being activated as soon as the boiler shuts down on the thermostat. It could be either that the pump is worn and not getting the heat away from the boiler quick enough, or that the pump over run is not allowing the pump to continue to run for a while when the heating shuts off. If it is the pump, a standard heating pump will cure the problem, but if it is the pump over run then you will probably have problems getting the part for a boiler of that age. The Apollo is very much on a reduced parts availability.
Gas Technical Services
July 2008
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