flue

A flue is the void through which smoke, fumes and gaseous discharges pass, not the pipe or structure forming the void. The pipe is referred to as a flue pipe and a chimney is that part of the structure of a building which forms a flue. For example, the space within a brick-built chimney is the flue. Most heat-producing appliances must be connected to a flue, in the form of either a chimney or a flue pipe. Unflued appliances can lead to condensation trouble, but some oil-burning and gas appliances are designed to operate without being connected to any kind of flue. Some gas appliances do not connect directly to a flue, for example, a gas cooker.