Class 1 flue
The most versatile of all flues. It is 7 inches in diameter and is usually found in properties with conventional chimneys, which have, at sometime had open coal fires in them.
Most models of gas fire will fit into a Class 1 flue. You will generally find that gas fires with a high heat output or fires which are slightly less efficient will require a 7 inch flue in order to disperse the heat more effectively which is lost up the chimney.
Class 2 flue
A conventional flue but is only 5 inches in diameter.
The range of gas fires that can be fitted to a class 2 flue is slightly reduced to that of a Class 1 flue.
Usually found in more recently built properties.
The pre-cast flue is made from blocks, which fit within the cavity of an outside wall. Sometimes they are incorporated into a shallow chimney breast.
The flue runs up the side of the property and discharges in the same way as a conventional chimney but there is quite a restrictive range of fires made for this type of flue.
A Balanced flue
In a glass fronted fire a concentric flue projecting out onto an outside wall.
The flue has two layers, one within the other. One will draw air into the fire; the other will discharge the carbon monoxide outside to disperse into the atmosphere.
Usually fitted to an outside wall where no chimney is available.
Generally fitted to an inset open gas fire with no glass front. The fire takes its air from the within the room and a fan is fitted to the flue to assist in drawing air into the fire and pulling out the generated carbon monoxide.
Power flues need an electrical connection. If there is an electrical power failure, all powerflues have inbuilt safety devices that switch off the gas to the fire automatically.