Multifamily buildings with natural ventilation are still being built and exploited. Such buildings are often equipped with individual gas-fired water heaters located in windowless bathrooms. It implicates the possibilities of improper gas exhaust as a result of the decrease of infiltration, what could be sometimes even harmful for the occupants' health. Based on the numerical simulations, analysis of ventilating air flows in typicalmultifamily dwelling house will be carried out. it will be shown that effectiveness of natural ventilation in particular flats depends not only on the active factors determining infiltration phenomenon (like wind and temperature difference) but is also strongly connected wit flat location inside the complex structure of a building.