Guide to Conversion of Traditional Buildings

3.18 Combustion appliances – protection from combustion products

Mandatory Standard

Standard 3.18

Every building must be designed and constructed in such a way that any component part of each fixed combustion appliance installation used for the removal of combustion gases will withstand heat generated as a result of its operation without any structural change that would impair the stability or performance of the installation.

3.18.1 Application of standard to conversions

In the case of conversions, as specified in regulation 4, the building

as converted shall meet the requirement of this standard (regulation 12, schedule 6).

3.18.2 Commentary

This mandatory standard is concerned with the ability of chimneys and flues to safely remove the products of combustion, limit the effects of heat on the structural components and prevent the spread of fire into the building. Most traditionally constructed buildings will contain existing chimneys and flues, which may or may not be in current use. However, it is likely that the structural condition of many of these chimneys and flues will be unknown, and some may have deteriorated to the extent that they could allow the spread of heat, fire and combustion gases into the building.

Sometimes old flues were lined with ‘parging’, a lime render applied as the chimney was being built. This was often crudely applied and, over the years, may have become detached. Where historic chimneys and flues are to be retained in use for a conversion, it is likely that they will have to be upgraded to meet the requirements of this standard. However, it is not usually a difficult task to upgrade a chimney, provided it has been well maintained and is in a sound structural condition.

As part of the conversion of historic or traditional buildings, it is essential to conduct a detailed investigation of the condition of all chimneys and flues, not only for their ability to remove the products of combustion, but to assess the chimney for its structural stability and resistance to dampness. Dampness is a major cause of problems in chimneys, whether they are in use or redundant. Moisture combines with sulphates deposited inside the flue as part of the combustion process to form weak acids. The acids can attack the lime mortar in the parging or in the masonry joints. Moisture levels can build up rapidly in redundant flues where the fireplace has been built up or the chimney head capped. It is therefore essential to ensure that redundant flues are ventilated at their top and bottom.

a) Dampness in chimneys

Dampness in chimneys can be caused by:

i. The ingress of rain, which can enter directly down the flue and into the building, through defective flashings or through the wall itself due to the wall thickness being too thin or the pointing defective.

ii. Water ingress at the chimney head, where the mortar holding the chimney pot in place (flaunching) has decayed, allowing water to enter the chimney.

iii. Condensation within the flue (especially in tall chimneys).

Redundant flues can be put to positive use as part of the planned ventilation of a converted building. The flue can be adapted to act as ‘passive stack ventilation’, which may compensate for a more tightly sealed building elsewhere. This is a good reason to retain redundant flues and to keep them ventilated. A redundant flue can also be used to accommodate soil and vent pipes, and bathroom and kitchen vents, without disruption to other parts of the historic fabric. Where reusing redundant flues for this purpose, consideration should be given to the provision of adequate access to services installed within them for installation and future maintenance.

As well as issues relating to existing chimneys and flues, a conversion will often require the installation of new combustion appliances that are remote from an existing flue. The construction of new chimneys and flues, particularly modern metal chimneys, can be destructive to internal spaces and historic character when they are installed within the building. The location of a flue terminal requires very careful consideration if the character of the roof is to be retained. Inserting balanced flues for gas appliances through existing walls must also be carefully considered to avoid damage to both the fabric and character of the wall.

There is a special risk where a flue is formed in a rubble wall, as the moisture balance in the wall around the flue (especially in the case of tempered-earth mortars) may be affected, leading to excessive shrinkage as the wall dries out. Attaching a chimney to a building externally can have a significant adverse effect on the architectural and historic character of the building and should be avoided if possible.

It should be noted that according to the New Build Heat Standard (NBHS), in effect from 01 April 2024, new homes and buildings must install climate-friendly heating systems instead of oil and gas boilers. Wood burning stoves and peat heating systems can be installed in new build homes and buildings. The NBHS also applies to conversions that have a direct emissions heating system such as oil or gas-based systems. Where it is reasonably practicable to install a replacement clean heating system, wood burning stoves are permitted as a secondary or emergency form of heating.

Designers should aim to comply with this standard wherever possible. Installation of clean heating systems such as heat pumps often require the reconsideration of heat emitters. Care should be taken to determine risks to historic character or fabric as a result. For example, the installation of underfloor heat emitters may be incompatible with historic floor constructions and may inhibit moisture movement, as well as potentially causing damage during installation. Similarly, requirements for larger radiators may impact on the historic character of internal spaces. Feasibility studies should be carried out to determine whether such installations would be reasonably practicable without undue damage to traditional fabric or character.

3.18.3 Issues to be considered

Typical risks in the conversion of historic/traditional buildings

1. Redundant chimneys

• The removal of redundant masonry chimneys will result in the loss of historic fabric and change a building’s appearance and is therefore likely to impact on its cultural significance.
• Sealing up of redundant flues can lead to dampness within the flues and reduce room ventilation.

2. Dampness

• Poorly maintained chimneys can be a significant cause of dampness and structural decay of masonry.

3. New chimneys and flues

• New metal chimneys installed within buildings of traditional construction can be destructive to historic fabric and character.
• Modern metal chimneys fixed externally to the building can have an adverse impact on the cultural significance and character of the building.
• Poorly sited flue terminals can affect the character of historic roofs.
• Inserting flue terminals (balanced flues) through historic walls can result in loss of historic fabric and adversely impact the external façade and the wall construction.

3.18.4 Recommendations to meet the standard

Often, the only practical way to re-use an existing chimney flue-way in a traditional building is to install a flexible metal liner. This addresses most of the problems outlined in the commentary and is a suitable solution for most combustion appliances, from open solid-fuel fires to oil-fired boilers. The diameter and material of the liner must be considered in relation to the type of combustion appliance, and the flue may also have a positive or negative pressure designation depending on the appliance. Generally, the most suitable material is double-skin stainless steel with a positive pressure rating and, provided the correct grade of material is used, such liners would be suitable for multi fuel applications. BS EN 1856-1: 2009 and BS EN 1856-2: 2009 give the definitive guidance on this, and specialist flue manufacturers and installers will be able to provide advice based on the requirements of the British Standard.

Installing liners can be problematic. If the masonry flue is in poor condition, it may have partially collapsed in places and there may also be local partial obstructions, which narrow the masonry flue-way and so restrict the diameter of the liner that can be installed.

Recommended practice is to secure the assistance of a flue specialist to survey the flues by dropping mandrels of different diameters down the flues. If there is an obstruction which effectively blocks the flue or unduly restricts it, then a decision must be made as to whether it is necessary to remove the obstruction. This can be highly invasive, but if it is necessary to make the flue usable it may be the correct thing to do if the alternative to using the flue is even more invasive (such as running a new metal flue down the outside of the building).

3.18.5 Further reading

Additional guidance can be found in the following documents:

• British Standards Institution (2019) BS EN 1443:2019 Chimneys – General requirements.
• British Standards Institution (2009) BS EN 1856-1:2009 Chimneys – Requirements for metal chimneys, Part 1: System chimney products.
• British Standards Institution (2009) BS EN 1856-2:2009 Chimneys – Requirements for metal chimneys, Part 2: Metal flue liners and connecting flue pipes.
• Section 3.18 – Non-Domestic Technical Handbook (2026) and Domestic Technical Handbook (2026).