In article , Tim Mitchell Tim@sabretec
hnology.co.uk writes

The circles with Xs are gate valves. I haven't tried messing with that
one, I will work out its current state and try some different settings.
It is perfectly happy if the central heating valve is open. It bangs
worst if both CH and HW are open, then the CH valve closes, which makes
the boiler reduce its output to 1 or 2 lights, then it bangs for about
15 seconds, then it settles down and all is peaceful again.

Sounds exactly like my situation.

There are parallels here between fluid theory & electronic control theory
which I could bore the arse off you with, but suffice to say resistance to
flow damps oscillation. If that pump control loop is borderline stable, then it
may need some flow restriction to be guaranteed stable. The CH loop is
easy cos it has lots of little restrictions - long pipe runs and radiator
lockshield valves turned down to next to nothing. I think you will find that
closing down the HW loop flow a bit with the gate valve will improve the
situation. My 22mm valve is only open a half turn (of 8) for stability. I regard
this fix as a patch only & will be having words with Keston technical soon
in search of a permanent solution.
--
fred