The operational safety of power units depends upon many factors, including the methods of their operation, their working conditions, age, regularity and range of maintenance. The scope of the paper is the analysis of maintenance options for power machines and equipment. The assumed criterion for the selection of the range of repair works is the level of technical risk posed by a given facility below the accepted allowable level. Detailed discussion is focused on the water and steam system of the boiler. The influence of the maintenance on the probability of failure of certain components is described on the grounds of Kijima's model. For the assumed maintenance periods minimal sets of equipment were determined, the repair of which should secure the operation of the water-steam system for a successive interval with the risk level lower than the allowable one.