A method for estimating the reliability of mechanical ventilation systems in dwellings hasbeen developed. The analysis is based on component level reliability models interconnectedby so called fault-tree schemes. A simplified model for maintenance is included. Theanalysis procedure is applied on an central exhaust ventilation system and on a central supplyand exhaust ventilation system with heat recovery. For each system, three different qualitystandards have been defined and combined with three levels of maintenance. Work has alsobeen done on collecting relevant input data, e.g. expected life-time values for ventilationcomponents. The result of an analysis can be presented as a figure showing the reliability ofthe total system as a function of time. The result can also be presented in a compressed formgiving the mean and minimum reliability values for a certain time period. Finally, aclassification routine is proposed. This will transform the resulting mean and minimumvalues into a five graded classification scale.