Aquifer thermal energy storage (ATES) systems, which utilize underground water for heat exchange with buildings, have been proven to be an excellent heating and cooling source. However, their operation is limited by strict regulations, one of which is the requirement for balance in the amount of heat transfer to the ground. Systems are highly exposed to cooling dominated loads, which results in excess heat injection into the ground. Commonly, an air handling unit is utilized to expel heat from the ATES system. This is known as the direct compensation (DC) method. In this study, an alternative approach that uses night ventilation (NV) was presented as a promising solution in combination with DC. Night ventilation can be used to decrease the cooling load and by using NV the system can avoid excess heat injection into the ground. The DC method was combined with NV under various control settings and compared with a system that uses only DC. The optimal operational setting between DC and NV operation was determined based on simulating a case study building. The study determined that the energy performance of the system can be improved by 16% by optimally adapting NV to the DC method. (C) 2019 The Authors. Published by Elsevier Ltd.