The most commonly used central units in the district heating system are most certainly hot water boilers. Experience in exploitation as well as their former development have given rise to new demands and possibilities of using hot water boilers for achieving a more efficient utilization of fuel. Yet, the current trend is not only for an efficient operation in terms of energy efficiency of primary fuel, but it also comprises many strict requirements when it comes to a cost-effective, reliable and safe operation. A mathematical - empirical model of the hot water boiler during the start-up process is presented in this paper. The dynamic behavior of the object will be discussed in the start-up regime because it represents one of the most critical transient regimes during the operation