This document specifies, the requirements and tests methods concerning, in particular, the construction, safety, fitness for purpose, and rational use of energy, as well as the classification and marking of hybrid products. The hybrid product is composed by: a gas boiler as heat generator which could supply the heat demand in all operating conditions; an electrical heat pump, as heat generator, which has not to fulfil the heat demand in all operating conditions; a control unit (see definition 3.1.10). The gas boiler as part of the hybrid product covered by this document is a gas-fired central heating boilers from the types C1 up to C9 and the types B2, B3 and B5, according to the classification in EN 1749:2020: a) that have a nominal heat input (on the basis of gross calorific value) not exceeding 400 kW; b) that use one or more combustible gases of the three gas families at the pressures stated in EN 437; c) where the temperature of the heat transfer fluid does not exceed 105 °C during normal operation; d) where the maximum operating pressure in the water circuit does not exceed 6 bar; e) which shall be classified as gas boiler; f) which are intended to be installed either indoors or outdoors in a partially protected place; g) which may include the facility to produce hot water, either by the instantaneous or storage principle, the whole being marketed as a single unit; h) which are designed for either sealed water systems or for open water systems. This document provides requirements for boilers with known constructions. For boilers with any alternative constructions, which might not fully be covered by this document, the risk associated with this alternative construction needs to be assessed. An example of an assessment methodology, based upon risk assessment and which covers the essential requirements of the Gas Appliance Regulation UE/426/2016, is given in Clause 11. This document does not cover all the requirements for: Appliances that are intended to be connected to gas grids where the quality of the distributed gas is likely to vary to a large extent over the lifetime of the appliance (see FprEN 15502-1:2021, Annex DD); a) appliances using flue dampers; b) appliances of the types B21, B31, B51, C21, C41, C51, C61, C71 and C81; c) appliances incorporating flexible plastic flue liners; d) appliances designed to become connected to a combined flue duct system that is designed to operate under overpressure (for example Ca)). This document specifies minimum operating requirements which ensure that the products are fit for the use designated by the manufacturer when used for space heating and/or DHW production. This part specifies the common requirements and test methods concerning, in particular the construction, safety, fitness for purpose, and rational use of energy. This document is to be used in conjunction with: a) the gas fired boiler, the generic part EN 15502-1 and specific Part 2-1 and Part 2-2; b) the electrical heat pump, EN 14511-4:2018, EN 378-1:2016+A1:2020 to EN 378-4:2016+A1:2019 and FprEN 14825:2020; c) electrical safety, EN 60335-1:2019, EN 60335-2-102:2016, EN 60335-2-40:2003, EN 60335-2-40:2003/A13:2012 and EN 60204-1; d) for domestic hot water production, EN 13203-5.