The purpose of decentralised solar district heating plants is to feed solar heat directly into district heating networks. This decentralised heat supply has to consider two major output conditions: a stable required feed-in supply temperature and a feed-in heat power equal to the heat output from the solar collectors. However, many installations cannot achieve the second output condition, since severe oscillations appear in the feed-in heat power. This problem can be solved by two different control concepts with either temperature- or flow-control. Detailed measurements from two reference plants are provided for these two different control concepts. One main conclusion is that a robust control system is characterized by the ability to provide required flows and temperatures. The major difference between robust and less robust control is that the supply temperatures and/ or flows do not fluctuate even if the input conditions are unfavourable.