Cross-sectional reductions for existing chimneys are used for renovation and/or modernisation. The purpose may be to upgrade the chimney for operation under wet conditions or adapt the cross section of the chimney to a newly connected heating appliance, e.g. a low-temperature boiler. A cross-sectional reduction can affect the flue gas velocity. Application-specific calculations for correct flue sizing therefore form the basis for cross-sectional reductions.

Regulatory framework

In this work area, DIBt grants:

• national technical approvals (abZ) for flue liners and/or sections of flue liners for cross-sectional reduction that are not covered by the applicable harmonised standards or a European Technical Assessment;
• general construction technique permits (aBG) for the execution of cross-sectional reductions not covered by the execution standard DIN 18160-1.

National technical approvals and general construction technique permits can be granted together in a single decision ('combined decision').

European Technical Assessments (ETAs) can also be issued for construction products for cross-sectional reductions which are not (fully) covered by a harmonised technical standard. For construction products covered by an ETA, a general construction technique permit for the application may be pertinent.

The technical rules to be observed in the execution of cross-sectional reductions include:

• DIN EN 13384-1 – Chimneys – Thermal and fluid dynamic calculation methods – Part 1: Chimneys serving one heating combustion
• DIN EN 13384-2 – Chimneys – Thermal and fluid dynamic calculation methods – Part 2: Chimneys serving more than one heating appliance
• DIN V 18160-1 – Chimneys – Part 1: Design and performance

Please also note the relevant provisions of the federal states following MVV TB, Annex 8, Section 8.3 (2017/1) or Annex 14 (2019/1 and following).