Earlier field measurements in Low Energy Buildings have shown that excess temperatures can easily occur during summertime in well-insulated houses, also in northern part of Europe. If a ground source heat pump is used for heating and there is a bidirectional ventilation system, the borehole can be used for free cooling in summertime and the chilled air can be distributed by the ventilation system. In this study, a simulation of a single family nZEB located in the Swedish city Gothenburg was conducted. Several different cases investigating the effect of window opening, ventilation air flow rate and installation of a free cooling system was simulated. As expected, the simulation showed that an increase of the ventilation flow and opening of windows can be efficient means to increase the thermal comfort by lowering the number of hours with temperature exceeding 24°C. However, the results show that the free cooling system reduces the number of hours with high temperatures even more, especially for cases when the windows are not opened. To validate the simulation results, a prototype free cooling system was installed in a real nZEB, with the same properties as the nZEB in the simulation study. The measurements confirmed that it is possible to lower the indoor temperature considerably by free cooling by use of the borehole and supply the air system, even though the cooling capacity is limited due to restrictions on ventilation rates and supply air temperature. Another conclusion of the measurements was that the control of the free cooling system is crucial to achieve the full cooling potential and for high system energy performance.