In article , Tim Mitchell Tim@sabretech
nology.co.uk writes

Mine is open vented, not a sealed system, so there is no pressure gauge,
and I don't have any trv's either!
Ah.

However I believe that you are
correct, that the pump is pulsed at low speed and it's that pulsing on a
pocket of air which is causing the hammer.

I think the air, if there is some, is probably in a long horizontal run
of pipe which goes from the boiler at the back of the house, underneath
the upstairs floorboards to the airing cupboard where the motorised
valves are. Can't seem to shift it though and it is not preventing the
water circulating.
Ok, have you tried one rad or branch on at a time at max demand?, max
flow in a branch may shift your bubble to a rad for bleeding. Sure, boiler
cycle won't stay at max for long on only one rad turned on, but try
repeating. Also try one at a time with lockshields fully open - sorry,
rebalance time.

An alternative view is that there was air before I drained it, which was
damping the system, and now there isn't any the pump is making the pipes
twitch. In this case I can probably resolve it by fixing the pipes down.
Yup

Good luck
--
fred