A passive house is a cost-efficient building that can manage throughout the heating period, due to its specific construction design, with more than ten times less heat energy than the same building designed to standards presently applicable across Europe. This paper describes the thermal performance during the cold season of the AMVIC passive office building, located in Bragadiru, a small Romanian town 10 km south of Bucharest. A detailed description of the building structure and the HVAC equipment is made. A time-dependent model (PHTT - Passive House Thermal Transients) is developed and used. Models validation is performed by comparing the outputs with results by the Passive House Planning Package (PHPP) developed by Passive House Institute of Darmstadt. Two renewable energy sources are used during the cold season within the building. First, passive solar heating is provided by the large window on the facade oriented south. Second, a ground heat exchanger (GHE) increases the fresh air temperature. Results show that the GHE is the most useful and reliable renewable energy source from November to March, providing heat during the day and the heat flux increases when the weather is colder. The passive solar heating system provides a large part of the heating energy during the cold season. Classical building heating is necessary mainly during December-February. (c) 2010 Elsevier Ltd. All rights reserved.