This paper presents a simulation of a solar water heating system with cylindrical tubes. It can help students to understand thermal transients of simple systems. The model allows to simulate cylindrical pipes and tanks exposed to the air, the wind, the sun’s rays, throughout the year, during the day and night, based on the consumption of hot water, the size of the tank. The mathematical model is also used to evaluate the solar radiation by means of two sensors of temperature. Results are obtained in MS Excel and VBA (Visual Basic for Applications) macros. Transient equations are solved with the explicit forward differences method.