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Flue Installation


Guide to Natural Draught Flueing

[ Last Updated 20 August 2007 ]


Within this section…

To ensure good flue performance over the life of the flue, the installation of a flue should take into account the following:

  • elimination of slots or gaps to avoid the possibility of entrainment of air into the flue other than through draught diverters,
  • protection of combustible material by distance or insulation,
  • proper joining and mechanical integrity,
  • provision of adequate support and protection from potential future damage,
  • provision for removal of condensates, where applicable7, and
  • provision for connection and disconnection of the flue from the appliance in such a way that it does not damage integrity of flue system.

The lapping direction for flue components should be downwards if the appliance produces condensation.

Masonry flues (chimneys)

In addition, installation using existing masonry chimneys should ensure:

  • the structural integrity and soundness of existing chimney for conversion to gas,8
  • removal of any moisture entrapped in an unlined chimney previously used with solid fuel or oil burning appliances so that it does not adversely affect its structural integrity,
  • provision for avoiding any hazards associated with falling material (debris), and
  • allow for the larger thermal mass of a masonry chimney, to avoid the possibility of spillage as such flues can take longer to heat up.

Single skin flues

Single skin flues are more prone to heat loss and, as there is no outer skin, soundness of joints is more critical. Therefore, design and installation of single skin flues has to ensure:

  • heat loss is minimised,
  • joints are sound and do not create restrictions, and
  • protection of combustible materials through isolation or insulation, for example when penetrating floors.

Double skin flues

Double skin flues have better insulation properties than single skin flues and have lower heat capacity than masonry chimneys. While arguably not as critical as in the case of single skin flues, attention to jointing, heat loss and protection of combustible materials are all important factors in good flue systems.

Flue Terminal

The flue terminal is an important component of any flue system in protecting against adverse pressure conditions and blockages caused by foreign materials.

Poor design and location of flue terminals can lead to poor flue performance, especially in adverse conditions. Therefore their design and location must:

  • avoid location in close proximity to openings in the building and in other structures, to avoid possible ingress of combustion air or impingement of hot products of combustion;
  • ensure flue termination provides clearance in all directions to avoid adverse pressure effects from adjacent structures usually by extending the height of the flue,9
  • ensure draught has minimal effect on flue pressure– See Figure D,
  • protect against hazards due to activities of birds and vermin – eg by use of a properly sized mesh,

Figure D: Flue Terminals

Figure D: Flue Terminals

  • provide sufficient strength and support to the flue system, and
  • provide adequate corrosion resistance.

7 This may be the case, for instance with high efficiency appliances or where the environment is such that condensation is likely. A mixture with any soot that may be present can result in a mildly acidic (and therefore corrosive) solution.

8 There is potential for masonry structures to develop leaks over time due to the effects of ageing and movement of the structure.

9 Adverse pressure patterns and turbulence can readily arise from adjacent roofs, ridges, parapets or even adjacent structures),


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