Interest in waste water heat recovery in the residential and commercial sectors has increased significantly in recent years. Integration of heat pumps with these systems introduces the possibility to use the recovered heat freely. However, integration of heat pumps also makes these systems more complicated. For the system designer, this means that knowledge regarding the influence of design parameters on the system performance over both the short- and long term is needed. This seminar presentation looks into how different parameters influence the system performance when using heat pumps; a few examples of heat exchangers are also introduced.