The main goal of this article is to determine the optimal time period of boiler for biomass renewal, because its usage affects the financial situation of the firm in the form of costs. In this contribution is applied the method of operational research - the method of optimal time of machine renovation. The results of the analyses point to the optimal period of biomass boiler renovation before its service life in the 11th year. Operating costs are increasing about 9% after this time. By using this model there is ability to plan financial sources for buying new biomass boiler at 5500-15600€ on the market. The scientific benefit lies in the fact that the model is applicable to all types of equipment and machinery, in various industrial areas and in services and it is preventive instrument for financial planning for machine renovation.