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(Adam) wrote in message . com...
Hi All,

We have a rapid knocking coming from our hot-water tank. It happens
at some strange times too - I have noticed it a few times - once late
in the evening, once at around 0430 and this morning. On at least one
of these occasions (0430) the hot water *should* have been off.

My first assumption is that it is caused by the water boiling, and my
first assumption would be limescale were it not for the fact that
Edinburgh has very soft water and that the system is sealed. Any
ideas?

I know it's bad form to follow up one's own posting, but more info...

The knocking occurred at least twice last night - once around 1130,
once around 0230. The hot water was turned off at the wall thermostat
for the boiler.

There is, however, a switch in the hall near the boiler that says 'hot
water' or similar with a red light embedded below the switch itself.
Switching this off caused the knocking to tail away. Switching it
back on caused the knocking to return after a few minutes. Now, that
sounds like an electric element heating water to a boil to me...

Have we got electrically heated water in addition to the gas boiler?
When we first moved in and switched the water on (at the boiler
thermostat), we were expecting nearly instantaneous water because we
believed we had a combi, but there was no hot water until we switched
this 'hot water' circuit on (and then we had to wait a half hour or
so). Has this been installed in the past because someone couldn't get
the boiler to heat water? Most importantly, can I find out what in
installed in my house without paying a fortune to get a plumber in to
suck his teeth and tell me I need to spend a fortune on replacing it
all?

I forgot to bring the manual for the boiler into identify it - it is a
Vokera, though. I'll try to remember to do that tomorrow...

Any advice gratefully received...

Confused - Adam...