Costs are one of the main decision factors for the selection of domestic ventilation systems.This often leads to a ventilation system that just meets the requirements of buildingregulations at the lowest initial costs. Decision makers are often not aware of the impact of thequality of the ventilation system on life cycle costs, not only for the ventilation system itselfbut also for the building, as a result of complaints or even damage due to a poor functioningventilation system. In Annex 27 a simplified tool is developed to compare costs of ventilationsystems, not only regarding the initial costs of the system, but also costs of maintenance andcosts of complaints and failures related to ventilation and ventilation systems. Little is knownabout these costs, specially costs of maintenance and complaints. To collect some data asurvey was carried out amongst 19 housing corporations, owning about 100,000 homes, andrepresentatives of the Dutch ventilation industry (VLA).The first step was to put up a framework for the tool. With data and information of the Dutchventilation industry and four housing corporations, owning buildings and homes withdifferent types of ventilation systems (ME and MVHR), types of maintenance activities aswell as maintenance cycles were defined. The second step was to estimate a relation betweenthe user frequency, basic quality and the expected maintenance level, depending on the type ofventilation system. As a prerequisite it was assumed that a certain maintenance is necessary tomaintain the performances (IAQ) of the ventilation system. The third step was to collect dataon costs for maintenance and complaints amongst the other 15 housing corporations. Itappeared from this part of the research that very few corporations were aware of the necessityof planned maintenance and the costs for maintenance as well as the costs for complaintsrelated to the quality of the ventilation system. Despite this it was possible to "fill" the toolwith data on costs by comparing these data with more common data on total maintenance.The simplified tool can be used for estimating the expected maintenance and maintenanceactivity cycles as well as the total life cycle costs for the system, the maintenance of thesystem, the expected complaints and the ventilation related maintenance of the building.The last step that is to make is linking this tool with the reliability tool of Annex 27. SveinRuuds (1) and Johnny Kronvalls (2) work on reliability and system safety analyses is used asa basis for this.