In article , Tim Mitchell Tim@sabrete
chnology.co.uk writes
At the weekend I drained the central heating and refilled it because I
was getting some simmering sort of kettling noises from the boiler. This
fixed that noise, it's really quiet now, but sometimes when the boiler
is running on lowest speed (Keston Celsius 25, which can vary its pump
speed) I get a repeated banging noise from the pipes (water hammer type
noise) where they pass through the floor upstairs. There is no noise
from the boiler itself. I can hear a slight swishing in the pump as it
runs at low speed and the bangs match up with the swish. I am wondering
if I have introduced an airlock somewhere (as it did not do this
before), and the compressible air is causing some sort of pressure wave.
The pipe run under the floor is not fixed at all (crap Barratt
handiwork), sounds like it is twitching and banging the joists or the
floorboards.

Everything is getting hot as expected, so it's not too serious, any
ideas on what to do? My thoughts are-
a) drain it again (don't really want to do this)
b) try to clear the (possible) airlock somehow
c) clip the pipes down properly

Does the pressure gauge on your system fluctuate in time with the swish?,
mine does. I have the same boiler and have been fighting an instability in
the flow since installation. My thoughts so far are that the flow from the
pump is not as smooth as it could be - pulsating somewhat. If there is
anything spongy in the system eg. air, then this can turn the pulsing flow
into an oscillation which in some circumstances can build to a hammer. In
these circumstances, my pressure gauge will be doing it's nut!

In my case I found that the pulsating pressure was being stored (and
released) in the expansion vessel causing the TRVs to bang open & shut. I
believe my problem lies in some a couple of subtle design issues, which
don't help you, but it suggests that the keston system is susceptible to
flow instability.

All I can suggest for you right now is looking for air somewhere. Also,
when you hear the hammer, go round tweaking and listening at the TRVs
to see if it is actually one of them making the noise - a starting point at
least.

My own design problems have been patched temporarily by adding a flow
restrictor to the expansion vessel and by adding a slightly open manual
bypass.
--
fred