"Simon Elliott" Simon at ctsn.co.uk wrote in message
.. .
After a new boiler was installed, our CH system started knocking after
it had been running for a while. Turning the thermostat on the hot
water cylinder up higher would make the knocking stop for a while. The
guys who did the installation tried turning the pump speed down, which
helped a bit but didn't fully solve the problem.

The chap who came round to service the boiler suggested that it might
be washer flutter in one of the thermostatic valves. Today being a cold
day, the knocking started again. I went around the radiators with
thermostatic valves, and found one which seemed noisy. Turning the
valve slightly stopped the noise. Playing with the valve, it seemed to
be quite clunky.

Is there anything I can do with this valve without draining the whole
system?

Yes - leave the valve fully on or fully off.

I have the same problem, since installing a new boiler. I suspect one or two
valves were put in the wrong way around, but it did not cause a problem with
the old pump. The new pump more powerful and so is causing the valve to
knock. Next time the system is emptied, I'll turn the valves around, but in
the meantime, leaving a few valves fully on is not too big a problem.

-- JJ