This European Standard specifies, the common requirements and test methods concerning, in particular the construction, safety, fitness for purpose, and rational use of energy, as well as the classification and marking of gas-fired central heating boilers that are fitted with atmospheric burners, fan assisted atmospheric burners or fully premixed burners, and are hereafter referred to as "boilers". This European Standard is to be used in conjunction with the specific Parts 2 (Part 2-1 and following ones). This European Standard applies to boilers of types B and C, according to CEN/TR 1749:2009: a) that use one or more combustible gases of the three gas families at the pressures stated in EN 437; b) where the temperature of the heat transfer fluid does not exceed 105 °C during normal operation; c) where the maximum operating pressure in the water circuit does not exceed 6 bar; d) which can give rise to condensation under certain circumstances; e) which are declared in the installation instructions to be either a "condensing" boiler or a "low temperature boiler" or a "standard boiler". If no declaration is given the boiler is to be considered a "standard boiler" f) which are intended to be installed in a partially protected place; g) which are intended to produce hot water either by the instantaneous or storage principle, the whole being marketed as a single unit. This European Standard applies to boilers designed for sealed water systems or for open water systems. This general standard and the specific standards (see Part 2) provide requirements for boilers with known constructions. For boilers with any alternative constructions, which might not fully be covered by this standard or a specific standard, the risk associated with this alternative construction shall be assessed. An example of an assessment methodology, based upon risk assessment, is given in Clause 11. This European Standard is not intended to cover appliances intended to being connected to gas grids where the quality of the distributed gas is likely to vary to a large extend over the lifetime of the appliance.