It is well known that store elements such as liquid tanks, water reservoirs, or gas containers, are basic elements in heating, ventilation and air conditioning (HVAC) systems development and implementation. In a wide range of applications they serve as efficient energy storage elements by capturing heat energy amount in hot water form. This paper presents a LabView graphical software-based energy-flow model development for store elements in HVAC systems. In the first research steps the mathematical background describing the energy-flow processes in the considered store element (water tank) it is presented and discussed. Then an original software model has been designed and implemented to simulate the ongoing energy-flow processes. The result of the above mentioned research efforts is a powerful and versatile software toolkit (virtual instrument) well suitable to modeling and simulate complex energy-flow processes in HVAC systems embedding various types of store elements. Beside the elaborated mathematical model concrete software simulation examples and measurement data are also provided in the paper. Not at least, the proposed original model offers a feasible solution for future developments and research in HVAC systems software modeling and simulation purposes.