A model of a storage tank with an immersed serpentine heat exchanger is described and validated against experimental data available from the literature. The tank is modelled one dimensionally using the multi-node approach corrected by an energy conservative reversion elimination algorithm to prevent inverse gradient solutions to occur. A one dimensional model in the flow direction is also used for the serpentine based on control volume techniques. The serpentine is discretized in equal sized control volumes and the energy equation is solved in each of them. The energy exchanged between the serpentine and the tank is then introduced as an internal heat source of the tank multi-node. With this model the behaviour of tanks with internal serpentines can be predicted minimising tuning parameters to be derived from previous experimental analysis of the tank. Additionally, by an appropriate formulation of the governing equations in the serpentine control volumes, it is possible to handle complex internal fluid phenomena as coupling of the tank within a thermosyphone cycle or two phase flow. Peer Reviewed