The present invention relates to a control method of preventing backflow of exhaust gas into combustion chambers of boilers or water heaters that are not in operation for a multiboiler comprising a plurality of boilers or water heaters connected in line, each of which including a wind pressure sensor measuring wind pressure of air flowing into a combustion chamber and a controller controlling a fan supplying air into the combustion chamber on the basis of the wind pressure measured by the wind pressure sensor. The control method includes measuring wind pressures using the wind pressure sensors in the operating boilers and calculating an average wind pressure, using any controller as a master controller to determine a control amount of wind of the fans of non-operational boilers based on the average wind pressure, and operating the fans of the non-operational boilers on the basis of the control amount of wind.