When the weather turned colder I turned up the heating but the boiler started making kettling noises and then banging - it turns itself off and restarts later, with the same noises.
I thought there might be a blockage somewhere reducing water flow (pump and diverter valve are only a few years old and I'm pretty sure working ok) but the Vaillant boiler is ~10 years old, while the pipes and radiators more like 30 years old. When fitting a new pump a few years ago I refilled with a protector solution like Fernox F1.
So I drained the system - found a lot of brown water in the header tank and managed to remove a layer of sludge on the bottom of the tank. Then refilled with added Fernox F3 cleaner. I got all the air out of the radiator bleed values, and after refill I was able to get each radiator to heat quite well when just one was on at a time, so probably all pipes are unblocked. But with just a few radiators on I get the banging noises from the boiler at an even lower boiler temperature setting than before, so the system is barely producing usable heat. The F3 seems to have made the problem worse, and I'm not sure what to do next. Obviously it has to be flushed out in a few days in any case.
Would refilling with F1 inhibitor plus an anti-noise fluid like Sentinel X200 be enough to solve the problem?
Or should I get someone in to do a power flush and system clean and refill?
Or what about MagnaCleanse, which seems to be a sort of system flush, with MagnaClean filter permanently installed to trap iron particles?