I had a new bolier fitted yesterday and the plumber told me that it makes no difference if I leave the hot water setting on the programming unit to 'On' all the time and there is no point in using the timer. He said that having it set to 'On' will not be using energy, unless we are using hot water, i.e the system stops heating when the water gets to a certain level and temperature and will only kick in (even if 'On') if water is used. If we are out during the day and not using hot water, leaving it 'On' will not be doing anything or using energy.

Is this true and if so, what's the point of having a timer anyway