Balanced flues are designed to allow gas and other types of fire to burn fuel in a sealed chamber. They consist of concentric pipes, i.e. one pipe within a larger pipe. The air necessary for combustion is drawn through one pipe, and carbon monoxide and other waste gases are discharged through the other pipe. so avoiding direct contact between combustion and the air in a room.
A balanced flue connects from the fire or boiler to the outside air, passing through the external wall of a property. While the location of the flue can be quite flexible, gas regulations stipulate that the external end must be placed in such a position that carbon monoxide is not drawn back into the building through open windows, doorways or other ventilators.
The use of balanced flues enables boilers and fires to be installed in properties without requiring traditional chimneys, and also enables modern boilers to be installed in older homes without needing to connect into an existing chimney.
The flue must not be allowed to become blocked in any way, nor must there be any leakage of carbon monoxide from the flue into the interior if a house. A RICS Level 3 Survey will indicate if there is any obvious damage to the flue or if it is wrongly positioned, but a full test of the gas system may be required to show if the flue is functioning properly.