A balanced flue appliance is a sealed unit drawing air for combustion directly from the outside of a building and discharging the waste gases of combustion through the same external terminal. To achieve this, a twin wall, concentric flue system

is connected to the appliance and uses the outer tube to bring in the air whilst an inner tube expels the waste gases. The main advantages of balanced flue appliances are that they can offer greater flexibility in positioning within the home and provide improved heating efficiency compared to conventional flue models.