The water of appropriate quality introduced in the water system changes its chemical properties. Depending on the chemical properties of water and pipe materials, various phenomena may occur, e.g. corrosion, and biofilm structure. The decreasing water demand in existing water systems leads to a reduction in the water flow velocity in the pipes. Accordingly, the age of the water in the system increases. It is especially visible at connections and long sections of the network. The deteriorating water quality along with the elapsing time of its stay in the pipes makes it necessary to perform appropriate measures, e.g. flushing the network. Water supply services usually perform them intuitively. The choice of flushing sites, times, or flow rates is not measured and verified. The age of the water and the efficiency of flushing can be simulated in computer programs. EPANET provides tools for such simulations. The research aimed to check the effects of flushing the network and the age of water in the pressure reduction zone. This is a case that is particularly prone to the increasing age of water. The research has shown that the network flushing sites used so far contribute to the exchange of water in the main water pipes. The simulations showed the need for the additional flush in new places, and in the tested case, the age of the water in the pipes is as much as the intervals between subsequent rinses.