Residential and commercial buildings play a significant role in energy consumption sector. Buildings account for approximately more than 30 percent of total energy consumption [1] and this fact proves that energy management in buildings should be prioritized among governments and energy experts. Combination of heat pumps with seasonal thermal energy storage in order to meet the heating and cooling demand has been considered in recent decades. As it mentioned above, significant advantage of implementing seasonal energy storage is to meet the mismatch between the energy demand season and energy storage season. The main aim of this research is based on the concept of supplying multi-family houses space heating and cooling demands by implementing water-source heat pump which is connected into a buried sensible seasonal storage tank. The considered seasonal storage tank is a low temperature tank (30 °C, 86° F) as opposed to a solarassisted type. First phase of the project is consisted of charging the tank during summer time by incorporating shell and tube heat exchangers by which the waste heat from the condenser of refrigeration cycle will be transferred to a secondary fluid (water) in order to charge the storage tank. The amount of stored heat will be used in winter time in order to supply the heating demand in evaporation side of the heat pump (discharging). By implementing this type of combination between heat pump and seasonal storage tank, the mismatch between heating generation and heating demand can be highly covered. The other remarkable advantage of this combination is the reduction in compressor electricity consumption and COP enhancement. In this research, a high-resolution transient system modeling software is mostly utilized to achieve reasonable results in transient mode. In order to analyze and assess properly and fairly, the results and outputs are compared to a conventional system which is commonly utilized in HVAC industry. Here an air-source heat pump has been chosen and assessed in order to compare it with the combined heat pump system with seasonal storage tank.