Based on the analogy between electricity and heat, we find that electric transformers and heat exchangers have many similarities. However, unlike transformers where Vo can be higher than Vi, the outlet temperatures of conventional heat exchangers are limited (i.e. Tc,o<Th,i, Th,o>Tc,i). Such limitations make it difficult to identify and evaluate thermal equipment or systems for many applications. For example, in city central heating systems, in order to reduce the flow rate in the primary network, Th,o is often expected to be lower than Tc,i. In this case a traditional heat exchanger cannot meet this requirement. How to conveniently identify or evaluate the most suitable system for such applications is an important but unsolved problem. In this paper we propose a new concept, the heat adaptor, to address this problem. The idea is to combine heat-work conversion equipment (heat engines and/or heat pumps) with conventional heat exchangers. Using this concept, a typical process of a heat adaptor is established and its thermal performance is analyzed. The results show that the best process and best device arrangement can be obtained for given conditions, and that the thermal performance of these processes can be evaluated.