Balanced flue or Conventional flue?
Conventional chimneys rely on the natural circulation of heated air to expel the products of combustion up your chimney and can work with either open or glass-fronted fires. Air for combustion is supplied from the room the gas fire or stove is installed in.
Balanced flues work in conjunction with glass-fronted fires only. Here the appliance is sealed from the room in which it is installed and a twin-wall pipe vents directly outdoors. Air for combustion is drawn in through the outer pipe whilst the inner pipe removes the combustion gases to the exterior of your property.Depending upon the fire or stove selected, the twin-walled pipe may exit horizontally through an external wall or vertically through the roof.
Details courtesy of Gazco