"xEcute" wrote in message ...
If you live in an hard water area the banging is almost certainly what is
known as "Kettling". This is caused by a build up of chalk/lime inside the
boiler.
Thanks for the comments but I am not sure they apply to me, I have
tried to explain why.

Well I do live in a hard water area *but* bearing in mind that the
CH/HW is pretty much a sealed system, unless there are leaks and thus
constant entrance of new water OR the system has been drained over and
over I am not sure that I can see how sufficient scale would get in
the heat exchanger. There will only be a finite amount of Calcium etc
in the water and even if the water is saturated the unless water
leaves the system an equilibrium will soo establish...won't it?

The build up reduces the water flow and volume with in the boiler
which will cause the water to boil thus producing the Kettling/banging. Some
time just reducing the water temperature or speeding up the pump will stop
the Kettling temporarily but if it is due to the furring up of the boiler
the only cure may be to replace the boiler.

Another reason I am not sure about this causing my particular problem
is that it only ever happens when the heating comes on. Is there any
reason that the kettling would not happen randomly whenever the
heating comes on? I think it is more likely that the water pressure
jolt kicks off the pump.

Perhaps it's all just wishful thinking and I need a enw boiler, any
other comments?

Tim