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 fluepipe to conduct gas, typically ventilation air or boiler exhaust. see Flue by convection (as with a wood burning stove or conventional gas fire)
- the gasses are expelled mechanically by a fan (as with balanced flueA combined flue, usually serving a gas heater or boiler, which handles both incoming combustion air and outgoing waste gas. 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 RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place). See more on the regulations , part JThe Approved documents, (England) part J, deals with Combustion appliances and fuel storage systems
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 usethis normally means that a building (or part of one) takes on a different use for planning purposes (eg a house is converted into a shop), but beware that it can have subtly different meanings legally, depending on who is using it. see Planning permissionthe legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which are all about whether the building is properly constructed). see more on Planning the legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which is all about whether the building is properly constructed). see more on Planning, for instance if you are splitting a building into two flats)
- flueblock chimneys
- connecting flue pipes
- repairing and reusing fluespipe to conduct gas, typically ventilation air or boiler exhaust. see more on 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
PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. 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.