Recently, small size once-through boilers are widely used in the industry as a source of heat due to their high level safety and simplicity of operation (It accounts for more than 95 % of the number of boilers shipped in Japan). As the interests in energy concerns and environmental problems increase throughout the world, the regulations related to the energy efficiency and the environmental footprint of the fuel burning equipment tend to be strengthened. Against this background, the performance of the once-through boiler improved considerably employing new technologies such as, higher thermal efficiency on the heat exchangers, combustion controls with smaller purge losses, improvement of the burner allowing low excess air operations, higher efficiency using the multiple-boiler control system, improvement of the condensate recovery system, etc… As far as the NOx emission is concerned, 65ppm (corrected to 0% O2) is achieved on oil (A-oil) fired boiler using the self-recirculation type burner, and 25ppm (corrected to 0% O2) on gas (natural gas) fired boiler using “Non-furnace low NOx combustion technique”. Lower NOx emission level can also be achieved combining other low NOx techniques to comply with more stringent regulations which can be found in some oversea countries.