The calculation of output of the Danish firm ``Aidt miljoe ApS`s`` marketed low flow solar heating system is described in detail. In September, 1990, a series of detailed measurements were taken on a prototype of an Aidt Miljoe low flow solar heating system. The system is described. A simulation model was developed for a small low-flow system with a covered container for heat storage and used for the calculation of the system`s output during the measuring period. This model is also described in detail. Calculated and measured production data are compared and the annual output of the solar heating system is then calculated. The compiled data are presented in detail. (AB).