This document 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 standard, low temperature and condensing central heating units equipped with a forced draught burner, or dual-fuel forced draught burner when operating on gas. These units comprise a boiler body and a forced draught gas burner brought together at the producer's assembly facility, the whole being designed and marketed as a complete appliance. The assembly on site comprising a boiler body complying with EN 303-1:2017 and a forced draught gas burner complying with EN 676:2020 and designed and marketed separately are covered by EN 303-3:1999+A2:2004. This document applies to types B23 and B23P boilers with a water temperature at normal operation not exceeding 105 °C and with a maximum water-side operating pressure not exceeding 10 bar. An alternative method to the test method applied to determine the useful efficiency is added for the units fitted with condenser (Annex B (informative)). This document does not cover boilers under the scope of the Pressure Equipment Directive (PED) which are covered by EN 14394:2005+A1:2008.