Operating flexibility is an important consideration in heat recovery network design. There are two basic approaches to the problem : the formulation of complex mathematical models which are solved using sophisticated numerical methods; and, the use of network simulation in order to evaluate and modify the proposed design. It is the second of the approaches that is used industrially. If this approach is to be improved, understanding of network function and behaviour is necessary. This paper provides such understandings and shows how they lead to procedures for the solution of flexibility problems. The methodology is demonstrated on a case study.