flow inside the building to air flow outside the building on the basis of at least the temperature of the outside air and of a load of the boiler taking a first air flow from inside of the building in accordance with the ratio taking a second air flow from outside the building in accordance with the ratio the second air flow being at a certain temperature and supplying a combined combustion air flow including the first and second air flows in accordance with the ratio to the boiler also an apparatus is provided for controlling combustion in boiler building permitting it to achieve the above steps application_number US 55909595 A description a boiler building for the part concerning the air flows and devices controlling them is shown schematically in fig1 combustion air is led to a boiler not shown through a combustion air duct c in the beginning of the combustion air duct c there is a mixing section 1 which is connected to air mass within the building and to air outside the building at an outside temperature determined by the meteorological conditions the first air flow q 1 from the outside and a second air flow q 2 from the inside is led to mixing section 1 through corresponding control devices 2 and 3 such as dampers by adjusting the control devices 2 and 3 a desired ratio of the above mentioned partial flows is reached whereby the amount of the air flow taken from the inside can be varied according to the situation without influencing the total combustion air flow introduced.