A mathematical model and numerical algorithms for simulating the heat loss (or heat gain) from insulated or bare hot water pipes is presented. The computer model, referred to as LOSTHEAT, calculates the temperature distribution in the water, pipe walls, and insulation as a function of time. The code is capable of modeling steady flow or no flow (standby) conditions. The steady flow condition can be specified over a relatively short time span so that almost any draw schedule can be modeled. With the ability to model a range of parameters, LOSTHEAT can be used for comparative studies such as establishing the heat loss differences between insulated and non-insulated piping, and calculating the effect of various pipe diameters on the outlet water temperature. The LOSTHEAT software was developed on a 486 personal computer using the MS FORTRAN 77 compiler.