Technical Study – Chimneys and Fireplaces: Design Considerations

What are the different types of chimneys?

Left: Custom built chimney example (Detail Library Detail DL246)
Right: Pumice flue liner from Schiedel

In contrast, system chimneys are ‘off the shelf’ products and have more limited design options. The requirement for detailed drawings will likely be limited to the key junctions with the building fabric. The benefit of specifying a system chimney is that the design and installation process is generally simpler and quicker as they are assembled and installed using standard prefabricated components and manufacturers instructions. As they are less labour intensive, the chimney is likely to cost less overall. There are two main categories of system chimneys: factory made metal chimneys and factory made masonry chimneys (often referred to as a flueblock chimney system). All types of system chimney need to be designed and installed in accordance with the manufacturers instructions and the systems should be BS EN certified.

Left: Masonry system chimney example from Schiedel (Isokern double module chimney system)
Right: Metal system chimney example from Schiedel (ICS twin wall insulated chimney system)

In new build houses, both custom built and system chimneys can be suitable options. The decision will come down to the budget, aesthetic preference, time constraints and the fuel type / combustion type of the appliance. The BMFCA (British Flue & Chimney Manufacturers Association) has useful guidance documents on their website to assist with design and choice of system.

It’s key that the chimney is designed to suit the specification of the appliance. Where appliances are not provided from the outset, the NHBC advises to design fireplaces and chimneys to suit the appliance most likely to be fitted.

What are the different types of appliances?

Appliances are often categorised by their fuel type, combustion type and installation type:

• Common fuels for appliances in new build homes include wood burning & multifuel or gas.

• Appliances can utilise open or closed combustion. In open combustion, oxygen to fuel the fire is drawn in from the room around the fire (so ventilation of the room is critical). In closed combustion, oxygen is drawn from outside of the building. Note that an open combustion system shouldn’t be installed in a residence with mechanical ventilation as the ventilation system may create a vacuum and risk smoke flowing back into the living area.

• Common installation types include freestanding, inbuilt or suspended. These can be open fronted or closed glass fronted. Closed fronted appliances are significantly more efficient and so would be recommended over open fronted alternatives or fully open fires.

What are the regulations?

The construction of fireplaces and chimneys is covered by the Building Regulations in conjunction with European and British Standards. These standards set out design criteria to ensure that chimneys will function safely. Some of the key documents to refer to are listed later in this post. Approved Document J is a good place to start and it should be reviewed before any design work takes place to ensure that all designs can achieve building control approval.

Below is a list of the key elements of chimney and fireplace design that are covered within Approved Document J:

Clearance from combustible materials – sets out the clearances required between the chimney / flue and surrounding combustible materials like wood framing, insulation, and roofing. This is to ensure that combustible materials are not at risk of catching fire.

Fire stopping – requirements for where a chimney crosses a compartment wall or floor to ensure that it doesn’t breach the fire separation requirements

Flue size – minimum diameters are given to ensure that the flue is sized to accommodate the volume of gases produced by the appliance

Flue outlets – requirements for flue outlets are given to ensure safe flue gas dispersal

Flue height – minimum heights are given for chimneys to ensure combustion products can discharge freely and will not present a fire hazard

Flue layout – restrictions over bends and openings in the flue pipe are set out to ensure flues offer the least resistance to the passage of flue gases

Fireplace recesses and walls adjacent to hearths – minimum dimensions and guidance on layout are provided to ensure that the building fabric is protected from fire

Decorative outer lining – requirements for the sealing of materials are given to prevent leakage from the appliance into the void behind the lining

Condensates – requirements are set out detailing that the flue should be insulated or be impervious, resistant to corrosion and have provision for the disposal of condensates. This is to ensure adequate control of water condensation within the flue

Access – minimum provisions for access of concealed flues are set out to allow for installation and servicing

Connections – design criteria for flue pipes that connect an appliance to a flue are set out

Ventilation – adequate ventilation is crucial for the chimney’s performance. Requirements are set out to ensure that the room where the fireplace or stove is located has sufficient airflow to support proper combustion and venting of gases

Hearths – requirements are set out for the depth, extent, material, visibility and location of the hearth in relation to combustible materials. This is to ensure that it safely isolates the appliance from people, soft furnishings and the building

Carbon monoxide – requirements are set out for the incorporation of an appropriate means of warning when carbon monoxide is released

Before detailing a custom built chimney, it is important to refer to the latest version of Approved Document Part J to obtain details of the requirements listed above. It should be noted that some requirements differ depending on the chosen fuel type.

Compliance with the wider regulations

It should be noted that chimneys and fireplaces can be heavy structures and require structural support to comply with Approved Document A. A structural engineer will need to design adequate support and restraint for the chimney.

Chimney and fireplace design also has relevance to the guidance within Approved Documents F & L. To ensure that the design complies with the requirements, enlisting the services of an M&E consultant or energy consultant would be recommended in addition to close liaison with product manufacturers.

Conclusion

Detailing the design of fireplaces and chimneys is a complex subject which requires close attention to the building regulations requirements. It’s important to remember that the choice of chimney type and appliance type will affect the detailed design requirements. Manufacturers guidance should be consulted from the outset to ensure that all products (both chimney and appliance products) are compatible and suitable for the desired use. A well designed chimney and fireplace will add value to the new home whilst ensuring the safety of occupants and the building fabric.

Detailing chimneys and fireplaces

Download the Details

Many of the details featured here can be downloaded from the Detail Library. For all our Chimney and Fireplace details follow the link below. They are available to all members, or can be purchased individually.

Recommended Reading

Recommended Reading

• Approved Document J

• Approved Document B (for guidance on compartmentation of buildings for fire safety purposes and for appropriate degrees of fire resistance for compartment boundaries)

• Approved Document F (for guidance on ventilation for health, and provision of extract ventilation using open flued combustion appliances)

• Approved Document L (for guidance on secondary heating appliances contribution towards space heating demand)

• BCFMA – Wood burning & multi fuel appliances in residential properties

• BS EN 1443:2019 – Chimneys. General requirements

• BS EN 15287-1:2007+A1:2010 – Chimneys. Design, installation and commissioning of chimneys – Chimneys for non-room sealed heating appliances

Image Credits

Source URL: https://www.dezeen.com/2021/11/20/ten-striking-fireplaces-lookbooks/

Image URL: https://www.dezeen.com/2021/07/22/authors-house-sleth-copper-clad-writers-studio/

Architect credit: Sleth

Photographer credit: Rasmus Hjortshøj, Coast

Source URL: https://www.drufire.com/en-gb/product-range/dik-geurts/ivar/ivar-5,-low

Image URL: https://www.drufire.com/getmedia/7903f8d7-27fe-4add-973a-92b36c7d283e/K53304_1050x700px_1_Dik_Geurts_Ivar_5_Low_EA

Architect credit: Unknown

Photographer credit: Unknown

Source URL: https://www.drufire.com/en-gb/product-range/dik-geurts/vidar/vidar-triple-small,-store

Image URL: https://www.drufire.com/getmedia/9d884e01-8ce2-41fb-a76f-f5b0f7b0b7eb/K53275-Vidar-Triple-Small-Store-1-Sfeer

Architect credit: Unknown

Photographer credit: Unknown

Source URL: https://www.dezeen.com/2022/11/26/suspended-fireplaces-lounge-areas-lookbooks/?li_source=LI&li_medium=rhs_block_1

Image URL: https://static.dezeen.com/uploads/2022/11/ca-lamo-maria-castello_dezeen_2364-1704×1217.jpg

Architect credit: Marià Castelló

Photographer credit: Marià Castelló

Source URL: https://www.dezeen.com/2020/05/06/three-chimney-house-t-w-ryan-virginia/

Image URL: https://static.dezeen.com/uploads/2020/05/three-chimney-house-t-w-ryan-architecture-virginia-usa_dezeen_2364_col_16-1704×1277.jpg

Architect credit: T W Ryan Architecture

Photographer credit:  Joe Fletcher

Source URL: https://www.schiedel.com/uk/products/pumice-system-chimneys/pumice-liners/

Image URL: https://www.schiedel.com/uk/wp-content/uploads/sites/12/2016/04/pumice-liners-page-header.png

Manufacturer credit: Schiedel

Source URL: https://www.schiedel.com/uk/products/pumice-system-chimneys/isokern-dm/

Image URL: 940002642 Isokern Brochure 30-06-21.pdf

Manufacturer credit: Schiedel

Source URL: https://www.schiedel.com/uk/products/steel-system-chimneys/ics/

Image URL: ICS Brochure 940002245 17-03-23.pdf

Manufacturer credit: Schiedel