For proper use of a new or a long time period operated boiler equipment, it is necessary to evaluate the effectiveness of its work, which is characterized by a coefficient of efficiency (efficiency). Its calculation requires greater accuracy as even tenths of its value can influence the decision on an application of one or the other boiler equipment kind or on a choice of the equipment producer. All the efficiency components can be determined experimentally (according to parameter chart of boiler equipment in operation) by means of modern instrument base, the exeption is the value of the heat losses from heated boiler surfaces into the environment or from the external cooling boiler surface. In the literature there are no information and recommendations on the choice of methods for this value determining. And for the small-capacity boiler equipment there are no even indicative figures. To solve this problem, it is necessary to study all the factors that affect the desired value, as well as methods for determining it. The results of experimental studies of the operation of the small-capacity boiler equipment allow us to identify the relationship between the amount of heat loss from the heated boiler surfaces to the environment, the thermal power value and the equipment surface area. Based on the obtained results, graphical and analytical dependencies are developed to determine the heat loss from the heated boiler surfaces to the environment.