Water heating in the U.S. has been identified as a major component of total energy consumption used in buildings, mostly coming from the residential sector at around 11%. A potential opportunity for energy savings in water heating systems is to improve the design of hot water distribution systems (HWDS). Due to the complex heat losses of HWDS, models are needed to optimize HWDS by reducing heat losses. There are three models currently used to simulate thermal performance of hot water distribution systems (HWDS): HWSim, ORNLHWDS, and TRNSYS. The first two models are used to study hot water distribution systems only and may not have interactions between a whole building and HWDS. The third model is a whole building approach and uses the “plug-flow” model to calculate the outlet temperature with variable size segments of fluid. The present effort is to develop a simplified HWDS model for a single pipe, which includes thermal mass of both fluid and pipe and can be u sed in the DOE-2 program as an input function and incorporated in other building simulation programs. The simplified model is described by a partial differential equation with time and axial distance as independent variables. The model is simplified enough to have an analytical solution and accurate enough to provide a good prediction of HWDS heat losses. The model was validated against measured data during both water heater on and off periods of a water heating system. Following this, an input function used in the DOE-2 program was developed based on the model. Simulation results show that the average domestic hot water heater energy use increases 3%, due to the inclusion of HWDS losses. Heating energy use decreases slightly and cooling energy use increases slightly due to a portion of the heat loss migrating to the conditioned space or adjacent spaces.