The concept of reusing wastewater and rainwater in buildings has recently become more important. There are countries where the use of greywater in some buildings has already been made mandatory (e.g. in Japan for toilet flushing). Such measures are gradually being introduced in the EU, where water recycling projects exist for office buildings and hotels. The Czech Republic still lacks experience and a design background in this area. The article is based on TAČR project No. TA01020311, titled “The use of grey and rain water in buildings”, and summarizes basic information about the quality of greywater, and methods of treating it and recovering its heat.

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The article presents current research results in the field of airflow through a façade with a width of 1 m and a height of 13.7 m and with a ventilated gap, and its effect on the year-round heat balance of this façade. An idea to influence airflow in the ventilated gap of the façade is presented based on the results of developed software and the suitability of closing the air gap in winter and in the transition period of the year is described. First, the boundary conditions of the calculations, which are further used in the energy balance between the interior of the building and the exterior environment are defined. In order to include these influences, a discrete analytical calculation was created. It consists of the time distribution of the investigated thermal phenomena calculations. A significant finding is an obvious benefit of controlling the airflow through a ventilated gap in the winter and especially in the transitional period of the year. This technological knowledge has a high potential for energy savings related to the heating of buildings. As the calculations show, airflow control through a ventilated façade reduces heat flow by 25–30% on average, and in contrast, it increases heat gains by 20% and the specific values are presented within the article.