This document gives details for the classification of gas appliances according to the method of supplying combustion air and of evacuating the products of combustion (types). This classification refers to gas appliances that are intended to be installed within buildings and/or outside of the building1. The European Standard classifies appliances as type A, B or C according to the basic principle for the evacuation of the products of combustion and air inlet. In references to a gas appliance/gas appliances connected via its or their duct or ducts, it is intended that the air inlet duct and/or the discharge duct for carrying any products of combustion are part of the gas appliance. This means that such ducts are certified together with the gas appliance. In terms of this standard a "single duct" is a flue duct designed and capable to discharging the products of combustion and/or air inlet duct for the air supply for only one appliance. In terms of this standard a "common duct" is a flue duct designed and capable to discharging the products of combustion and/or air inlet duct for the air supply for more than one appliance. This European Standard is a guide for the harmonization of product standards, for the preparation of installation standards and for the common understanding of the types of gas appliances. This European Standard is neither an installation standard nor a product standard.