My boiler doesnt seem to heat my water?

my glow worm boiler only seems to hear the water if i have the heating on. if i turn the heating down on the dial to 0 have a bath ect,, then the hot water goes cold and then doesnt heat up again untill ive turned the heating on, is this normal for this sort of boiler? or am i supposed to keep the electric water tank in the bathroom turned on?