The problem of achieving good air quality in dwellings whilst saving energy has led to a number of solutions many involving the use of fan power. This paper describes a programme of testing, that has continued over the last three years, of a passive low energy ventilation system. The systems components are supply air window in combination with passive stack vents. Installations have been monitored in unoccupied dwellings in Poland, Denmark and Ireland and user feedback is now being collected, the units now being occupied. Results are generally favourable, very low window Ue-values having been recorded in the buildings on the Continent. The retrofit houses being used illustrate the need for a good standard of air tightness and a well designed PSV system. It has been possible as a result of the data that has been collected to anticipate performance based on climate variables and the building specification and an interactive website is being constructed to make this tool available to potential users.