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If you can catch the system doing it you need to test (multimeter) for power to the pump during overheat (banging/boiling), and also feel if the pump is actually running. It could be pump failure or pcb failure. If both test to be sound then you may be looking at a faulty 3-port, I've seen the...

As the rapid temp rise is happening on both ch and dhw then I would say you have a blockage or restriction. And going on experience it will be at the bottom of the flow pipe from the primary heat exchanger where it elbows into the diverter valve. This is on the left of the boiler. The fibre...

Sounds like you have a sealed system and have ran out of pressure to completely fill the system. Have a look in the airing cupboard or near the boiler for a pressure gauge and a silver braided hose with taps to put water back into the system.

First check would be to make sure that the water flowing out of the hot taps is actually coming from the boiler. Isolate the cold inlet under the boiler and then open a hot tap to make sure there is no water flow. I've seen faulty showers (non return valves) feed water into the hot system so...