# Improving the Energy Efficiency of an Existing Building by Dynamic Numerical Simulation  
[PERSON]  
1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.C. )1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.C. )1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.C. )1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.C. )1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.C. )1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.C. )1 Faculty of Building Services, Technical University of Construction of Bucharest, 66 Pache Protopopescu Blvd., RO-02141 Bucharest, Romania; [EMAIL_ADDRESS] (R.S.P. ); [EMAIL_ADDRESS] (T.  
Existing HVAC (heating, ventilation, and air conditioning) systems using conventional fuels can be partially or completely replaced with 'green energy' HVAC systems. As a function of climate change over time, it is a great challenge for both architects and engineers to obtain an optimal solution which can achieve minimal energy consumption while also ensuring indoor comfort for new buildings. This goal is even more difficult to achieve for existing buildings, where the degree of freedom is considerably lower. Therefore, a way to approach the accomplishment of degree of energy consumption decrease through different methods is with the help of dynamic simulation tools. A good building refurbishment requires a proper combination between thermal insulation for different buildings' envelope zones (walls, ceiling, floor,...), better thermal resistance windows, but also renewable energy systems to make the building sustainable within its environment, a graphical representation of passive house's principles is shown in Figure 1.  
In order to be considered a passive house [7; 8], a new or retrofitted house should fulfill certain criteria, such as:  
* All opaque building components of the exterior envelope of the house must be very well-insulated, this means a heat transfer coefficient (U-value) of 0.15 W/(m\({}^{2}\)K) at the most;
* Window frames must be well insulated and fitted with low-e glazing filled with argon or krypton to prevent heat transfer, this means a U-value of 0.80 W/(m\({}^{2}\)K) or less, with g-values around 50% (g-value = total solar transmittance, proportion of the solar energy available for the room);
* At least 75% of the heat from the exhaust air should be transferred to the fresh air by means of a heat exchanger, allowing a good indoor air quality same time with saving energy;
* All edges, corners, connections, and penetrations must be planned and executed with great care, so that thermal bridges can be avoided. Thermal bridge care should be considered, as they cannot be completely excluded, but they must be reduced as far as possible;
* The space heating energy demand should not exceed 15 kWh/m\({}^{2}\) year (considering square meter of net living space);  
Figure 1: Basic structural principles for the construction of passive houses (with permission from [7]).