There are several types of flue fire, and each of them are different, so here is some information to help you understand flue fires.
Class 1 Flue Usually of masonry construction (identifiable by a chairbrick at the back of the opening). The flue must comply with the Building Regulations for Class 1 (solid fuel) appliances, with a minimum diameter of 175mm (7") and a minimum equivalent height of 3 metres. A Class 1 flue usually terminates with a brick or stone built chimney. Class 2 Flue Usually of metal construction and found within newer homes. Typically constructed using twin walled flue with a minimum diameter of 125mm (5") and a minimum equivalent height of 3 metres. Pre-Cast Flue Usually of pre-cast concrete block construction (hollow concrete blocks which form part of the inner skin of the dwelling) that terminates with a ridge tile. Balanced Flue A balanced flue fire does not require a chimney, just an outside wall. A flue arrangement passes through the outside wall behind the fire which removes the products of combustion and also allows in air for the fire to burn. Balanced flue fires are completely room sealed, highly efficient and have glass on the front through which the flames are visible. Power Flue A power flued gas fire uses a fan to remove the products of combustion from the rear or side of the fire to the outside of the house. Various flue configurations are available to suit the position of the fire within the layout of the property. Flueless Flueless gas fires are designed to operate without a flue. Catalytic flueless gas fires can be installed on any wall, with or without a fireplace opening, in almost any room with a minimum room size of 30m3. Because all the gas is used to heat the room and none is lost up the chimney, these fires boast 100% efficiency.