Abstract: A vertical hot water storage tank with an access opening in its underside, for positioning an internal heating element in the tank, and connectors for the supply of cold water to the tank and for the egress of hot water therefrom, is in the form of a pressure tight cylindrical vessel. In order to simplify the construction of the vessel, it is assembled from separate modular elements made of a plastics material, which are relatively adjustable to determine the capacity of the vessel and which are finally bonded together. The modular elements may comprise a pair of inter-fitting top and bottom lid elements, or top and bottom lid elements interfitting with a central tubular element. The capacity of the vessel may be determined prior to the bonding of the elements together, by the extent to which they overlap one another or by suitable dimensioning of the elements.