In the method for detecting the configuration of a ventilation system, the exhaust air valves in the premises are detected. The ventilation system comprises a control apparatus (122) and a first subsystem (100), which includes a first ventilation unit (102) with an exhaust fan (104). The first subsystem further comprises a first exhaust air duct (108) connected to the exhaust fan, the ends of the exhaust air duct having been led to the premises (120, 121, 220, 221) and an exhaust air valve (112, 212) having been installed into the ends. In the detection, the exhaust air valves are determined an individualizing identifier, the exhaust fan of the first ventilation unit is switched on, the exhaust air valves installed into the first exhaust air duct are adjusted to the first flow position, and reference pressure values are determined for these exhaust air valves by measuring pressure differences over said exhaust air valves. One exhaust air valve connected to the first exhaust air duct in the first premise is then adjusted to the second flow position, in which air flow through the valve is substantially bigger than in the first flow position, pressure differences are measured over the exhaust air valves in the first flow position and the measured pressure differences are compared with the reference pressure values of the said exhaust air valves. The closed exhaust air valves, the measured pressure difference of which deviates substantially from the reference pressure value, are determined as valves in the first premise and they are given the same premise value as the exhaust air valve in the second flow position. The identifiers of the exhaust air valves with the same premise data are transmitted to the control apparatus. The premise can be an apartment of a block of flats or a terraced house.