Latent thermal energy storage (LTES) heat exchangers can improve a wide range of energy systems, especially in regard to an increasingly intermittent energy supply. For each of these applications, LTES heat exchangers need to be designed and sized. Designing an LTES heat exchanger is equivalent to predicting the outlet temperature of the heat transfer fluid as a function of the inlet temperature and mass flow rate of the heat transfer fluid for a given heat exchanger geometry. Design methods for LTES systems are limited to sizing methods which predict averaged outlet temperature or using full scale CFD calculations which are computationally expensive. A less computationally intensive method is using analytical models for design. However, these models are limited to specific geometries and require severely simplifying the behavior of the heat exchanger. The present paper decomposes an LTES heat exchanger into a series connection of three sub-heat exchangers. The first heat exchanger is a system which only contains the heat transfer fluid (HTF) present in the LTES heat exchanger, the second part is a heat exchanger with only sensible heat and finally a third heat exchanger which is dominated by the latent heat. For each of these three sub-heat exchangers, analytical solutions are proposed. By connecting the three heat exchangers in series, the outlet temperature of the complete LTES heat exchanger is approximated. The paper develops the novel decomposition and compares the outlet temperature prediction to a numerical simulation of a planar LTES heat exchanger. The decomposition method obtains good agreement with the numerically predicted outlet temperature.