Mike Smith wrote:
Hi,

I have an open-vented, S-plan system with two 2-port zone
valves, 28mm to hot water cylinder and 22 mm, dropping to 15mm,
to radiators. There is an automatic bypass valve fitted
in a loop-back circuit.

Last winter I started to get a banging noise from the central
heating system. The noise was a load bang in the pipework
that occured intermittently shortly after the heating cut out.

I think this is described as water-hammer?

As it happened, I was in need of a new boiler anyway, and arranged
for British Gas to come and install a new boiler, pump and zone
valves. (Pause to wait for traditional BG insults). The idea here
was to change everything that could cause the water hammer.

The radiators in the system (13 mixed sizes) were newly fitted
about 3 years ago.

So, BG went ahead gave the system a chemical clean, and installed
a 300+ HE boiler, new pump and new bypass valve but, due to some
admin cock-up, new zone valves were not fitted, so I'm left with
the old ones.

When I complained about this, BG told me that there is nothing wrong
with my existing zone valves and that the replacement bypass valve
should fix the loud-bang-as-zone-valve-closes problem.

Sadly, this turned out not to be the case, and I am still
getting the load bangs with the (mostly) all-new system.

Would any CH guru out there care to suggest what is
causing the water hammer and what I need to get BG
to do to fix it? Is it possible/likely that the zone
valves are to blame or am I fixating on them unnecessarily?

I'm currently waiting a call-back from BG and would like to
prepare myself with what options there are to diagnose
and fix this problem.

I wonder if you might have an old directional zone valve that has been
fittted the wrong way around? It may be that with a bit of slack developing
in the actuating mechanism, the ball in the valve is bouncing on its seat?

Work out which way the water is flowing though your valves (feel for which
side heats up first when the system is turned on) and make sure that this
agrees with the direction indicated by the arrow (if present) on your
motorised valve.

Of course, if it was agreed that BG were supposed to replace the valves
anyway then get the idle buggers back to do the job they were paid for.

Tim