Reducing the world usage of fossil fuels and the CO2 emissions from its combustion, are among the main goals of the world’s governments.While in 2011 the world’s total primary energy supply was 13113 Mtoe (basically oil, coal and natural gas), in Sweden was 49 Mtoe (nuclear, oil, biofuels, waste and hydro). Therefore, Sweden is trying to use renewable and clean energy instead of the fossil fuels. District heating (DH) is one of the possible ways to produce heat, and in 2010, Sweden used 45 TWh from this energy carrier. It is an environmental and efficient type of energy, due to the fuel’s flexibility (Biofuel and Waste accounts for 70% of the fuel used for DH). It is mainly used in residential and services (91%), while the rest is used in industry (9%). It is the most important energy carrier delivered for heating purposes, representing the 50% of the total heating demand. While in multi-dwelling buildings and non-residential premises DH accounted for 86% and 69%, respectively, for one- and two-dwelling buildings the corresponding proportion was 10%. The project uses DH as energy carrier, and studies the costs of two possible heating system designs for a one-dwelling building in Sweden. A comparison between the investments and the determination of how both cases affect the Legionella growth are the aims of this project. This thesis could be beneficial for those who want to know the background of the house’s heating system. The two mentioned ways are the actual one, that is being used nowadays (the space heating and hot tap water system are independent); and the second one, which is an implementation of this, consists of joining the circuits. Thus, the potable water for the production of domestic hot water will be the working fluid for the space heating circuit. With this change the heating system is reduced, using just one heat exchanger for both space heating and hot tap water, while in the conventional configuration there are two (one for each circuit). Due to the fact that the piping and radiator system destined for the space heating will work with normal tap water (which has dissolved oxygen, which is an important iron corrodant) the elected material for the alternative system will be copper. The material used in the conventional space heating system is steel. Due to this differentiation in the material, the costs for both cases differ; being more expensive for the alternative design than for the conventional one. While for the first case the cost rises to 224600 SEK, for the second one the cost is 179700 SEK. These costs include all the material and installation costs. Since in this thesis, both systems consider the production of hot water as instantaneuos (not storaged water), the possibility of Legionella positive specimens is non existent and so both are free of this bacteria. Therefore, it can be concluded that it makes no sense at all to implement the alternative case for the heating system. From Legionella growth point of view, the possibility of positive specimens in the water is non-existing for both systems studied. From an economical point of view, the reduced system would imply a 25% of increase in the investment, with respect to the conventional heating system.

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