On Mon, 19 Jan 2015 13:46:32 +0000 (UTC), Andrew Gabriel wrote:

If you have a very predictable load, say, 3 showers every morning,
you can ensure the cylinder is heated up ready. OTOH, if the load
is very unpredictable, away half the week but different days, only
one person, etc, then you will end up wasting some of the hot water
and/or maybe not having it when you want it.

Maybe but when the time switch on the CH/HW (open vented cylinder)
failed I took a while fixing it, then noticed that the gas bill had
dropped so didn't bother until I moved out. The cylinder would only
take 20 mins to reheat and would stay hot enough for washing hands
and dishes for the best part of 48 hours. Running the system under
"on demand" rather than a time switch certainly saved money.

--
Cheers
Dave.