A friend has a problem with water hammer, that she showed me today. It's
the usual one-off "thump" when the tap is turned off.

The arrangement seems to be that there is no hot tank, the hot water being
supplied on demand (and presumably at mains pressure) from a combi boiler.
The system was installed as part of the central heating system about a
year ago. There is an upstairs header tank in the loft which I assume
feeds the cold supply to the bath, wash basin and toilet. I suspect there
used to be a water tank with immersion heater before the new system was
installed.

The water hammer occurs every time that the hot tap is turned off - bath,
wash basin or downstairs sink. It doesn't occur at anytime the cold water
is used. I don't think that it occurs when the cold tap is used in the
kitchen. The kitchen taps go through a mixer and this is hard to turn off
suddenly in one go. The bath and wash basin taps are mixer taps with a
single lever that has a left/right movement for mixing and an up/down
movement to turn the water on/off. This can cause the water to stop
immediately.

Although this only seems to occur from the hot taps, I assume that it also
occurs on the cold water main taps (I forgot to ask if it happened with
the washing machine.

She called in a plumber who replaced the ball valve in the header tank,
but that made no difference (I'm not sure why that was changed, given that
the it happens from other sources, but I assume that the plumber knew what
he was doing.

Any suggestions as to what could be done?

Thanks

Roger
(my reader sometimes loses mail/newsgroup messages
- if you think you should have had a reply/comment,
please e-mail me again. Ta!)