In this paper, an algorithmic-evolutionary synthesis procedure for retrofitting heat exchanger networks is proposed. It consists of two stages. First, after the amount of maximum energy recovery(MER) is computed, a grass-root network featuring minimum number of umits(MNU) is synthesized. In this stage, a systematic procedure of synthesizing MNU networks is presented. It is based upon the concept of pinch, from which networks are synthesized in a logical way by the heuristics verified by the pinch technology. In the second stage, since an initial feasible network is synthesized based on the pre-analysis result of MER and must-matches, an assignment problem between new and existing units is first solved to minimize total required additional areas. After the existing units are assigned, the network can be improved by switching some units and splitting streams. For this purpose, improvement problems are formulated and solved to utilize the areas of existing units as much as possible. A couple of exampes are used to demonstrate the effectiveness of the proposed method.