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Flues are pipes or ducts which carry waste gasses away from a building to the air outside. This can work in two different ways:

  • the exhaust gasses are hot and rise naturally up the flue by convection (as with a wood burning stove or conventional gas fire)
  • the gasses are expelled mechanically by a fan (as with balanced flue gas boilers)

With balanced flue boilers there are clear limitations and instructions which come with the boiler concerning how the flue should be installed. The maximum length of flue varies considerably from model to model. Some need to be positioned so that the flue goes directly through the wall to the outside while others can have several metres of flue which allow the boiler or heater to be positioned on an internal wall.

While the term ‘balanced flue’ has normally been used for gas and oil boilers, recent developments have made it possible to include wood pellet stoves, which have small balanced flue outlets on external walls rather than the conventional vertical chimney type flue. See the Swedish Ariterm Arrow flue outlet

In the case of convection driven appliances there is a whole range of flue materials and configurations which are possible. The main considerations are covered in the Building Regulations, part J

This particular Approved Document can serve as a pretty good guide for flue design as it covers all the main aspects which are to do with safety along with quite a few detailed construction drawings.

Areas covered include:

  • masonry chimneys and the types of flue liner they require. (also covers change of use, for instance if you are splitting a building into two flats)
  • flueblock chimneys
  • connecting flue pipes
  • repairing and reusing flues This includes testing old flues and types of liner
  • factory made metal chimneys
  • running factory made chimneys through combustible areas such as floors and ceilings, cupboards etc
  • bends in flues
  • openings for inspection and cleaning
  • low level flues near boundaries
  • notice plates for hearths and flues

Passivhaus stove flues

There have been a considerable number of fires in Passivhauses with class 1 heating appliances (such as wood stoves). These have been attributed to heavily insulated and sealed roof areas where heat buildup has occurred, possibly due to flue fires. An article in Green Building magazine, summer 2013 describes one such very serious fire, which although the flue was twin-wall and to current building regulations. pretty much burnt the house down. A Dutch company, Metaloterm, manufacture a high grade insulated flue designed to prevent such fires.

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