After trying to balance my system as best as possible, I'm pretty sure that its balanced properly. I still have the problem though that I'm getting banging from the boiler towards the end, and after the burn period.
I don't ever remember this happening prior to replacing the boiler thermostat (where it was turned off for 3 days or so). I'm pretty sure that after that, the banging noise started. The heating worked for a few days until I then replaced all the rad valves with TRVs and flushed all the radiators of sludge. I didn't use any flushing agent before doing it though (so the pipes would still have sludge in) and I realised that when refilling, the sludge at the bottom of the F&E tank was disturbed by the water filling the tank and therefore would have been introduced into the system. I put in inhibitor of course when refilling!
I'm not 100% sure the noise came about after replacing the thermostat or after the TRVs (though I'd say 80% or so).
From my new IR thermometer, the boiler flow temp is just under 80 degrees when thermomstat on max, and the return is about 10 - 15 degree drop depending on whether its heating or not heating the water at that point in time. All radiators are nice and hot with very similar return temps.
The boiler is banging loudly towards the end of its firing period and for 30 seconds (guess) after the burner is off but room stat is still calling for heat.
Until the boiler stat failed, I don't remember hearing any noises from the boiler. When the boiler stat failed, it boiled water intermittantly for a while (up to a max of an hour or so but don't remember hearing the noise of it more than twice.). Could this have produced any significant increas in scale on the heat exchanger? The boiler is old (old Baxi WM 531 RS boiler) and I've got a sneaky feeling that the old drain c**ks leaked (they were literally covered in blue/white crusty bits and wouldn't turn on or off and felt wet - possibly like this for years). Based on this leaking, the apparent lack of boiler servicing, I suspect that the inhibitor may have been diluted and fresh water being present, so scale build up may have been possible over time. Would actually boiling the water caused more rapid scale build up or even blockage of parts of the heat exchanger? Why is it suddenly doing it now, when it didn't do it before?
The pump seems to work okay (quite noisy on higher speeds but I don't think its a flow problem) and increasing pump speed does not appear to alter the banging noises, nor does changing the boiler stat setting.
Based on all this, does this sound like scale/blockage of the heat exchanger? Could the sludge I introduced when filling the F&E tank blocked the heat exchanger?
I'm thinking that I should try some boiler descaler (the type you can keep in there afterwards) and then, if the noise hasn't gone after a number of weeks, maybe flush with some de-sludging stuff and then replace inhibitor again.
Sound sensible?
Thanks
D