The energy needed for the production of domestic hot water represents an important share in the total energy demand of well-insulated and airtight buildings. Domestic hot water is produced, stored and distributed above 60°C to kill Legionella pneumophila. This elevated temperature is not necessary for domestic hot water applications and has a negative effect on the efficiency of hot water production units. In this paper, system component models are developed/updated with L. pneumophila growth equations. For that purpose different existing Modelica pipe and boiler models are investigated to select useful models that could be extended with equations for simulation of bacterial growth. In future research, HVAC designers will be able to investigate the contamination risk for L. pneumophila in the design phase of a hot water system, by implementing the customized pipe and boiler model in a hot water system model. Additionally it will be possible, with simulations, to optimise temperature regimes and estimate the energy saving potential without increasing contamination risk. Keywords: domestic hot water (DHW), Legionella pneumophila, pipe model, boiler model, contamination risk, energy use Attachment Annex 1-Approach 1 to determine multiplication time (y and yb) Annex 2-Approach 2 to determine multiplication time (y and yb) Annex 3-Explanation of conservation equations Annex 4-Pipe and boiler models: reasons for non-retention Annex 5-Simulation model components used in pipe and boiler component Annex 6-Understanding the simulation log basics

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The energy needed for domestic hot water represents an important share in the total energy use of well-insulated and airtight buildings. One of the main reasons for this high energy demand is that hot water is produced at temperatures above 60°C to mitigate the risk of contaminating the hot water system with Legionella pneumophila. However, this elevated temperature is not necessary for most domestic hot water applications, and has a negative effect on the efficiency of hot water production units. A simulation model has been developed which proposes an alternative to this constant 60°C by predicting the Legionella pneumophila concentration dynamically throughout the hot water system. Based on this knowledge, a hot water controller is added to the simulation model that sets a lower hot water comfort temperature in combination with heat shocks. In this paper, the simulation model is used to estimate the energy saving potential in a case study building, at the level of the heat production system by reaching higher production efficiencies. Three different production units, namely an electric boiler, heat pump and solar collector have been investigated. The controller is expected to become an alternative for the current, energy intensive, high temperature tap water heating systems.