A case study of the first nearly zero energy office building (nZEB) in Rakvere, Estonia was conducted to determine whether an office building can be built without a conventional space heating system while ensuring adequate thermal comfort in the offices. Energy and indoor climate simulations of alternative solutions were carried out and the feasibility of the solutions, ensuring heated rooms throughout the year, was assessed based on investment cost and payback calculations. The results showed that despite of the low heat losses, a nZEB still needs a space heating system with room based temperature control. Heating needs primarily occurred during weekends and at night; however, without space heating the air temperatures in the rooms dropped down to 16.7 °C during occupancy and were below 21 °C during about 700 occupied hours. Supply air heating with variable air volume system, controlled according to the coldest room and on demand night operation, was able to keep + 21 °C temperature in all rooms, but resulted in significant energy penalty caused by overheating of offices with lower heat losses and increased fan electricity use. The economic analysis showed that a building with simple constant air volume ventilation system and radiator heating was most feasible. The investment cost increase of the variable air flow ventilation system was too high compared to the savings in energy cost that was already low.
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