how does my old Backerloy Heater work

The top heater is used to heat the top of the tank only eg for washing up. The bottom heater heats the whole tank eg for baths.

You have off-peak / White Meter / Economy 7 and the bottom heater is using cheap-rate electricity at night?

Do you have electric heating?

Owain

If you look at you will see your immersion heaters in the left hand drawing as #3 & #4. If you then read the "stored hot water" section lower down you'll see reference to heating your water overnight. (you may not have a system that exactly mirrors the diagram but in respect of two immersion heaters and a tank of hot water "it'll do"). Each of your heaters will have a thermostat under their metal caps to limit the hot water temperature and somewhere I would expect you have a time switch to switch the bottom element on at night. Something like this

this

Thanks Owain and Peter. Your information helped a lot and I now understand what is happening. Oliver