Dangerous levels of the Legionella bacteria, which causes potentially fatal Legionnaire's disease, were discovered in the Scottish parliament in 2009 for the second time. A number of infected showers were sealed off and taps dismantled in Queensberry House and the Canongate building [1]. It was only the one of the latest recorded cases over the world. The interpretation of results is also difficult; a negative result is no guarantee that Legionella bacteria are not present. Conversely, a positive result may not indicate a failure of controls, as Legionella are present in almost all natural water sources. There are a lot of guidelines and regulations developed in many individual countries for the design, operation and maintenance for tap water systems to avoid the growth of bacteria Legionella pneumophila. The aim of this paper is to show the necessity of regulations for installations inside buildings by use of the simulation methods.