An air conditioning equipment of a building for return air admixture of off air into ingoing air, which equipment comprises - a speed controllable ingoing air fan in the ingoing air flow channel arranged to regulate requested ingoing air flow; - a speed controllable outgoing air fan in the off air flow channel arranged to regulate requested outgoing air flow; - a section for return air flow controlled by an adjusting means to regulate return air flow; - a control and regulating system which comprises a monitoring and calculation unit arranged, based on one or more sensors present in the buiding to individually regulate first and second air flow means (14, 15) and speed controllable fans (11, 12) to provide a desired return air admixture; characterized in that - said speed controllable ingoing air fan is placed In the air flow direction subsequent to the section for return air flow; - said speed controllable outgoing air fan is placed in the air flow direction subsequent to the section for return air flow; - said first air flow means regulates addition of outdoor air, and is placed in the ingoing air stream in the air flow direction prior to the section for return air flow; - said second air flow means regulates the return air flow, and is placed in the section for return air flow, whereby - said first and second air flow means (14,15) are individually regulated in sequence such that when said second air flow means is regulated to a required degree of opening, the first air flow means has a full degree of opening, and when said second air flow means has a full degree of opening, then said first air flow means is arranged to provide a required degree of opening to balance the need for outdoor air flow and return air flow, and In that - means for measuring ingoing and outgoing air flows are arranged at inlets of the speed controllable fans (11, 12) present in the air conditioning equipment for monitoring ingoing air flow and one or more of the following air flows, outdoor air flow, outgoing air flow, or return air flow, which means are arranged to provide output signals to the monitoring and calculation unit to control the outgoing and ingoing air flows determined. - a speed controllable ingoing air fan in the ingoing air flow channel arranged to regulate requested ingoing air flow; - a speed controllable outgoing air fan in the off air flow channel arranged to regulate requested outgoing air flow; - a section for return air flow controlled by an adjusting means to regulate return air flow; - a control and regulating system which comprises a monitoring and calculation unit arranged, based on one or more sensors present in the buiding to individually regulate first and second air flow means (14, 15) and speed controllable fans (11, 12) to provide a desired return air admixture; - said speed controllable ingoing air fan is placed In the air flow direction subsequent to the section for return air flow; - said speed controllable outgoing air fan is placed in the air flow direction subsequent to the section for return air flow; - said first air flow means regulates addition of outdoor air, and is placed in the ingoing air stream in the air flow direction prior to the section for return air flow; - said second air flow means regulates the return air flow, and is placed in the section for return air flow, whereby - said first and second air flow means (14,15) are individually regulated in sequence such that when said second air flow means is regulated to a required degree of opening, the first air flow means has a full degree of opening, and when said second air flow means has a full degree of opening, then said first air flow means is arranged to provide a required degree of opening to balance the need for outdoor air flow and return air flow, - means for measuring ingoing and outgoing air flows are arranged at inlets of the speed controllable fans (11, 12) present in the air conditioning equipment for monitoring ingoing air flow and one or more of the following air flows, outdoor air flow, outgoing air flow, or return air flow, which means are arranged to provide output signals to the monitoring and calculation unit to control the outgoing and ingoing air flows determined.