San Isidoro Royal Collegiate Church is an emblematic monument placed in the historic centre of Leon. A coal heating system, which was damaging the monument, was replaced by thermal energy production with biomass.The development of the project was made possible by the cooperation between the Chapter of San Isidoro Royal Collegiate Church, the Regional Entity of Energy of Castilla y León, The General Directorate for Cultural Heritage of the Regional Ministry of Culture and Tourism of Castilla y Leon (DGPC), representatives of the Intervention Plan of San Isidoro Royal Collegiate Church and the City Council of León.The investment of this initiative is approximately € 240.000 provided by the General Directorate of Cultural Heritage and Regional Entity of Energy. Biomass supply: Wood pellets are used from forest residues, which are supplied and/or made by Spanish companies, with an estimated annual consumption of 60 tonnes of biomass. Feeding: The supply of fuel from the tank to each one of the two boilers is done by an auger located inside the tank. A pneumatic transport system is responsible for carrying the pellets to the intermediate tank of fuel, located inside the boiler and with daily autonomy. Thermal power plant: Two biomass boilers (pellets) of 100 kWt each one were installed, with a high performance aspiration feeding. It also has control and automation elements which are needed to ensure an optimum combustion and minimum emission values.It has a pellet storage tank with a capacity of 16 m3 that allows storage of 10 tonnes of pellets, providing autonomy of 250 hours at full load.Thermal energy distribution: Inside the Basilica, custom-made banks with radiators have been incorporated in order to improve the comfort in the room. Moreover, new hot-air emitters for the Chapel and conventional radiators in the Sacristy have been installed.