Ah, we have a combiboiler, so no hot water tank, it just heats water when the hot tap is turned on.
Having it 'ticking' over constantly isn't economical, like the kettles that keep the water ready for use, a waste of energy for most households compared to boiling the kettle as needed.
It depends on how cold your house gets / how hot you want or need it to be and how long it takes to go from one to the other, as we have insulated the rads - we only have 4 in the whole house - on low works but others might not find it so.