Method for controlled admixture of a subset of exhaust air flow into a supply air flow at an air handling unit, which air handling unit comprises an exhaust air side and a supply air side, wherein the exhaust air side in the air flow direction comprises an exhaust air damper for control of the pressure of the exhaust air flow after the exhaust air damper, one mixing section, one heat exchanger for exchange of energy or for exchange of both energy and moisture between the exhaust air side and the supply air side, one speed-controlled exhaust air fan arranged in an extract air flow, and one extract air damper, and wherein the supply air side in the air flow direction comprises an outdoor air damper for control of the pressure in an outdoor air flow after the outdoor air damper, the heat exchanger, the mixing section, and a speed-controlled supply air fan arranged in the supply air flow, and the mixing section comprises a mixing damper for admixture of exhaust air flow in the supply air flow, and further the air handling unit comprises at least one first flow meter for measuring of the extract air flow and at least one second flow meter for measuring of the supply air flow, at least one first pressure sensor for measuring of a first pressure drop Δp 1 over the heat exchanger on the supply air side and at least one second pressure sensor for measuring of a second pressure drop Δp 2 over the mixing damper, one control and monitoring system for registration of measurement parameters and control of fans (9, 14), dampers (6, 11, 12, 15), and heat exchanger, characterized in that the method comprises the following steps: a) indication of needed admixture of exhaust air flow into the supply air flow, b) controlling and securing of positive pressure balance Δp 2 over the mixing damper, i.e. that the pressure is higher on the exhaust air side of the mixing damper than on the supply air side of the same, with the criteria Δp 2 > 0 Pa and a-b ≤ Δp 2 ≤ a+b Pa, where a is a selectable set value and b is a selectable interval value and a > 0 and b > 0, and whereby the balancing is performed by controlling either the exhaust air damper or the outdoor air damper, where which damper (6, 12) that is regulating is determined by the actual operating point, whereby if the second pressure drop is Δp 2 > a Pa + b Pa the exhaust air damper will close slightly at the same time as the exhaust air fan speeds up slightly to keep the wanted air flow, and if Δp 2 ≤ 0 Pa the outdoor air damper will close slightly, until Δp 2 fulfils the criteria Δp 2 > 0 Pa and a-b ≤ Δp 2 ≤ a + b Pa, c) the balancing is stopped when the criteria according to b) are fulfilled, or alternatively when a predetermined time for achieving correct pressure balance according to b) has expired, d) the mixing is started whereby the mixing damper opens at the same time as the outdoor damper closes, i.e. the both dampers (15, 12) works in parallel and in opposite phase, until a desired mixing ratio is achieved, and in parallel the exhaust air fan secures the correct flow in the exhaust air flow by speed-control. a) indication of needed admixture of exhaust air flow into the supply air flow, b) controlling and securing of positive pressure balance Δp 2 over the mixing damper, i.e. that the pressure is higher on the exhaust air side of the mixing damper than on the supply air side of the same, with the criteria Δp 2 > 0 Pa and a-b ≤ Δp 2 ≤ a+b Pa, where a is a selectable set value and b is a selectable interval value and a > 0 and b > 0, and whereby the balancing is performed by controlling either the exhaust air damper or the outdoor air damper, where which damper (6, 12) that is regulating is determined by the actual operating point, whereby if the second pressure drop is Δp 2 > a Pa + b Pa the exhaust air damper will close slightly at the same time as the exhaust air fan speeds up slightly to keep the wanted air flow, and if Δp 2 ≤ 0 Pa the outdoor air damper will close slightly, until Δp 2 fulfils the criteria Δp 2 > 0 Pa and a-b ≤ Δp 2 ≤ a + b Pa, c) the balancing is stopped when the criteria according to b) are fulfilled, or alternatively when a predetermined time for achieving correct pressure balance according to b) has expired, d) the mixing is started whereby the mixing damper opens at the same time as the outdoor damper closes, i.e. the both dampers (15, 12) works in parallel and in opposite phase, until a desired mixing ratio is achieved, and in parallel the exhaust air fan secures the correct flow in the exhaust air flow by speed-control.