The aim of this book is to contribute to more detailed analysis of heating energy extraction by the biomass combustion. This topic was chosen in reaction to actual EU energy policy requirements of increasing the consumption share of renewal energy sources. Combustion process, air pollution limits, fundamental combustion emissions and basic water boiler specification is described. This publication uses the knowledge from specialized publications, own experiences in the design and development of water based boilers, experimental measurements and cooperation with the industrial partners. Risk definition, risk analysis and risk evaluation techniques are described and applied in feasibility study on risk situation when no heating water is taken from the boiler. Biomass hot water boilers described in the book were developed, constructed and designed mostly by the authors and are commercially used within EU. The testing and measurements belong to authors' focuses. The measurements focus on operation safety and level of dominant pollutants emission. The results, sometimes giving surprising answers to fundamental questions, are presented in the graphs and tabs at the last part.