Taking into consideration that there is not definite differentiation between definitions "economy of energy" and "energy saving", often the effect of energy effective technologies can be received only by reduction of air exchange, that is breach of sanitary and hygienic regulations. In the article the attempt to distinguish these definitions with using system approach is done. In building branch absence of the system approach to building and city leads to serious system errors. For example, warming of building wallings using natural ventilation don't give real energy saving, but it leads to the air exchange decrease, therefore the given action raises risk of various diseases. In the article various system errors which can lead to acceptance of the erroneous decisions, influencing the sensitive initial data through air exchange reduction, are considered. A consequence of it are diseases of noninfectious character. To decrease quantity of system errors it is offered to use the system approach to these problems.