The present paper deals with optimizing a multi-energy district boiler by adding to the plant an optimally designed thermal water storage tank. First, an operation strategy is defined to manage the plant efficiently. Next, a reliable sizing method based on a parametric study is proposed. Various energy and economic criteria are evaluated for a range of thermal storage sizes. One can then choose a particular size that meets needs and expectations. Ones the size of the storage tank is defined, a simulation program allows the assessment of its impact on the boiler units dynamics. The proposed methodology has been applied to many multi-energy district boilers managed by Cofely GDF-Suez, our industrial partner. In this paper, we present a case study: a district boiler situated in southwest France. The results we obtained highlight the ability of a thermal storage tank (optimally sized and managed) to improve the operation of a multi-energy district boiler and realize significant economic savings.