Mechanical ventilation systems have a significant influence on the energy demand of buildings. The Government's aim is to reduce the energy demand for mechanical ventilation by 2020. Therefore, energy efficient mechanical ventilation systems are required. Using dampers to balance the ventilation system, the centralised fan must provide sufficient static pressure to overcome the maximum pressure drop in the system. This means that there will be excess pressure in parts of the duct system. Using decentralised fans instead of dampers, the excess pressure can be eliminated as each fan provides the pressure needed for that particular part of the duct system. In duct systems in conventional supply ventilation systems, there is a positive static pressure compared with the pressure inside the building. Using decentralised fans, parts of the duct system will have a negative pressure and room air may enter the duct system and spread to other parts of the building. In this paper, the possibility of reversed airflow was investigated. An evaluation of reversed airflow was implemented both theoretically and experimentally. With the objective of simulating a malfunctioning, one integrated fan did not operate during the investigations while the others did. When a manifold was used to distribute the pressure, the reversed airflow was eliminated. This was due to the positive pressure in the manifold that was maintained by the main fan. When the ventilation system was a regular supply duct system with a main fan and a main duct with branches, then there was a possibility of reversed airflow if a fan malfunctioned. (C) 2015 Elsevier B.V. All rights reserved.