A ventilation system for a building has a central venting duct for removing stale air or for supplying fresh air. The central duct (4) has a number of connecting side ducts (16) each fitted with a control valve and each connected to a different part of the building. The central position of the regulating valves enables the operator to set up the ventilation balance for the entire building from a central point. The control valves are designed for minimum flow resistance to the main flow through the duct. The connections to the side ducts include filters and connecting ducts.
