This diploma thesis deals with thermal calculations of fire tube water heater. The first part of the thesis describes different classifications of boilers and water heaters and provides a basic overview for different types of commonly used boilers. The next part describes thermal calculation model created in Maple software, used formulas and algorithm of the model. Subsequently, the model is applied to the experimental water heater in the form of computational rating. In the last part of the thesis the model is applied to the existing operated industrial water heater to evaluate predictive ability of the calculation model by comparing the essential operating parameters obtained from the calculation with real data measured on the water heater.