The double firebox boiler TERNA S and the double firebox +boiler tube TERNA B, are a logical addition to the heating system. The firebox of the TERNA S model is identical to the one installed in SIRIO for burning solid fuels such as wood, coal, compressed briquettes, etc. A second firebox has been installed next to this one, separated from the smoke side but communicating with the water side, that makes it possible to install a fan assisted gas or diesel oil burner to guarantee heating continuity when the solid fuel fire goes out.