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NEN-EN 1749 gives details for the classification of gas appliances according to the method of supplying combustion air and of evacuating the combustion products (types). This classification refers to gas appliances that are intended to be installed within buildings and/or outside of the building1.The document classifies appliances as type A, B or C according to the basic principle for the evacuation of the combustion products and air inlet.This document is the reference for the harmonization of product standards, for the preparation of installation standards and for the common understanding of the types of gas appliances.This document is neither an installation standard nor a product standard.In references to a gas appliance or 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 combustion products are part of the gas appliance. This means that such ducts are certified together with the gas appliance. Informative Annex C identifies appliance types that are designed for connection to separate chimney products, which may be part of the construction of the building.In terms of this document, a “single duct” is a flue duct designed and capable of discharging the combustion products and/or air inlet duct for the air supply for only one appliance.In terms of this document, a “common duct” is a flue duct designed and capable of discharging the combustion products and/or air inlet duct for the air supply for more than one appliance.