Should we leave our hot water on constantly, or on timer?

We’ve been told that leaving it on all the time hardly costs any more than just twice a dayEvery week a Guardian Money reader submits a question, and it’s up to you to help him or her out – a selection of the best answers will appear in next Saturday’s paper.Should we keep our hot water on constant, or on the timer? We have a factory-insulated hot water cylinder, and a friend insists it costs hardly anything extra to leave it on all the time (which we’d like) rather