Heater tank

A heater tank for an instantaneous water heater comprises a heater tank 1 having a fluid flow path 10 between an inlet 12 and an outlet 14 of the tank. One or more electric heating elements 16, 18, 20, 22 heats fluid (water) flowing along the flow path. The heating element(s) extend along at least a portion of the flow path such that the fluid (water) experiences a reduction in electric heating power as the fluid (water) flows along the flow path. Two or more of the heating elements may be arranged in series and at least one of the elements may comprise a plurality of segments (105a - 15t, fig 9) with the power of at least one of the segments being less than the segment immediately upstream thereof. One or more of the elements may comprise a coil internal filament having a variable helical pitch (fig 6) thereby varying the electric power. The flow path may comprise one or more parallel heater tubes 2, 4, 6, 8 each containing one of the elements. The tank may be part of an electric shower or plumbing system (wash basin, bath, dishwasher, washing machine) and reduces hot spots and limescale formation.