Energy Audit in two Multifamily Buildings and Economical Evaluation of possible Improvements : Case Study of District-heated Buildings with Mechanical Ventilation located in middle Sweden

Abstract: The energy performance of the buildings is important for economic and environmental aspect. The building energy use is gradually increasing. This is due to economical growth and increase in population. There are various energy efficiency measures that can be implemented provided they are economically feasible. Several Studies show that the implementation of energy saving measures such as thermal insulation results in more energy saving. However, most of the outlined measures are not economically viable. The paper outlines the energy efficiency measures through the use of the computer IDA ICE software. The evaluation of energy performance of two multifamily houses was conducted and possible suggestions such as thermal insulation, change of the windows, installing new air handling unit, installing heat exchange in the showers, improving the thermal bridges and change of schedules were presented. The economic feasibility of the suggestions is analysed using the life cycle cost to determine the economic viability of the energy measures. This involved determine the investment cost and life cycle saving cost to decide the best option. The most important factor in determining life-cycle saving is the modified uniform net present factor.  The addition of attic insulation, replacement of lighting bulbs, installing heat exchange in the showers and change of schedules, meet the economical requirement within the required time frame. The total energy saving from the measures that meet the economical requirement was approximately 71 MWh/year for Centralgatan 14 and 54 MWh/year of Tebogatan 5 corresponding to 24 % for Centralgatan 14 and 20.4 % of Tebogatan 5 reduction in heating demand. Replacement of lighting bulbs results in 5 MWh/year and 7.51 MWh/year  electricity energy saving for Tebogatan 5 and Centralgatan 14 respectively. The integration of energy measures has less impact on thermal comfort. The energy measures implementaed are within the acceptable limit of < 15 % PPD for thermal comfort apart from the combined effect of adding 200 mm external insulation plus lowering indoor temperature. Key word: energy audit, economical anaylysis, life cycle savings, investment cost, energy efficiency, IDA ICE