Domestic hot water and space heating demand in buildings contribute a high share of final energy demand. This demand is often met by fossil fuels, leading to large greenhouse gas emissions. Although renewable energy can be used for heating, there is time discordance between heat supply and heat demand. Seasonal thermal energy storage is a viable solution to overcome this mismatch. This paper presents a simulation method and a simple tool to assess the feasibility of integrating a seasonal thermal energy storage equipped with heat exchangers and/or heat pumps in a district heating system. The developed method and tool are generic and allow the simulation of hourly energy flows using energy balances and predefined conditions. In order to validate the proposed method, the result of the simulated energy flows from two selected systems, Friedrichshafen and Marstal, are compared with monitored values reported in literature. The comparison shows that while simulation of monthly energy flows depends on the accuracy of inputs to the tool, annual energy flows can be closely replicated. Hence, the method can be considered as validated. This simulation tool and method can be used to assess energy flows in a district heating system in future.