on my central heating, I can heat the radiators and/or the water either together or seperately. Well, i can in theory though often when just the water is on some of the radiators are hot too :o/
I also have a seperate immersion switch for use if i don't want thew boiler on at all, but i've never used it.
I emptied my hot water tank last night to have a bath, flipped the switch on the tank to heat it straight away, by the time I'd had a soak the water was hot enough to wash my hair, just over ½ hour later.