On Sun, 05 Nov 2006 19:56:22 +0000, Robin Nicholson wrote:

I have a modern immersion heater ( the green type with a jacket
integral) ...

Thats a description of the whole ho****er cylinder rather than just the
immersion heater which screws into the top or perhaps side, BWTH.

where I am and need to know whether it is cheaper to turn it on for an
hour when required or leave it on all the time. I am out most of the
day, most days and so do not draw much water.

The hotter something is the more heat it loses to the enviroment. As you
don't use much hot water I'd say you will be better off heating only when
required rather than maintaining the cyclinder temperature.

In my bachelor days the time switch broke on the CH/HW system, I couldn't
be bothered to replace it straight away and the boiler could heat the
cylinder from cold to hot in about 20mins. So I just used an "on demand"
system, the cylinder was well lagged and would stay useably hot for
washing for the best part of 48hrs. So instead of heating the water twice
a day and not using it, the water was only heated when I wanted it. The
effect on the gas bill was very noticeable, like about a 50% reduction.

--
Cheers
Dave. pam is missing e-mail