On Sun, 23 Nov 2008 04:26:09 -0800 (PST), Nick wrote:

When we use the hot water, it works ok for about 5 mins and then the
pilot goes out hence the flames go out. We have to wait a couple of mins
and then re-light the pilot after a few attempts. The hot water will
then work for about 3 mins and then same happens again and so on. The
central heating works fine and will carry on all night if necessary.

I'd be tempted to replace the thermocouple and check the size and position
of the pilot light flame. Though the fact that it works fine for CH is
counter to a duff thermocouple or badly adjusted pilot flame. Does the CH
use a different burner or different parts of the same burner compared to
HW?

Note what another posted has said about more noise in HW mode just before
it stops. It could be the overheat trip doing what it should due to the HW
heat exchanger being scaled up.

--
Cheers
Dave.