JultikaUniversity of Oulu repository

The aim of this thesis is to study and explain the purpose and the function of drain water heat exchangers. The thesis goes over theory behind heat transfer and heat exchangers and presents the general solutions of domestic drain water heat recovery systems. Systems gone over in detail are the different general shower drain water heat recovery systems. Another part of the thesis is a case study of an actual shower drain water heat recovery system of a Finnish household. The purpose of the case study is to study the actual temperature increase of cold water in a drain water heat recovery unit and efficiency of such heat exchanger. An alternate goal is to study the difference in efficiency values and temperature gains between two heat exchangers of the same model, where the other has been used significantly more than the other. In other words, another target is to study the fouling effect. The calculations are done using real measurement data. The most important findings are that utilizing a shower drain heat recovery unit provides real energy savings in the long run, and that there is a significant difference of efficiency between a dirty and a clean heat exchanger. Drain water heat recovery systems provided as high as 15 °C increase in the temperature of cold water. A clean heat exchanger boasts an impressive 50.4% efficiency, whereas the dirtier heat exchanger provides a 36.1% efficiency. The results can be further used to calculate the energy savings of the household on a yearly basis. Furthermore, the results show that domestic drain water heat recovery could potentially make a significant difference in national energy usage if implemented nationwide.