Guide to Conversion of Traditional Buildings

3.20 Combustion appliances – removal of products of combustion

Mandatory Standard

Standard 3.20

Every building must be designed and constructed in such a way that the products of combustion are carried safely to the external air without harm to the health of any person through leakage, spillage, or exhaust nor permit the re-entry of dangerous gases from the combustion process of fuels into the building.

3.20.1 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).

3.20.2 Commentary

The technical requirements for the construction and termination of flues from combustion appliances are very prescriptive in this mandatory standard and are well detailed within the Technical Handbooks as well as in the publications referred to within them. There can, however, be challenges in complying with these requirements when converting historic or traditional buildings. The notes below are intended to provide guidance on the range of options that are open to designers.

Two main categories of circumstance often arise:

a) Conversion to create several small units with independent heating systems, thus introducing several smaller boilers or other heating types (e.g. a building converted into flats). The New Build Heat Standard (NBHS) also applies to conversions that have a direct emissions heating system such as oil or gas bases systems. Where it is reasonably practicable to install a clean heating system, wood burning stoves are permitted as a secondary or emergency form of heating.

b) A relatively large, centralised heating plant for the conversion that consists of just one boiler, or other heating type.

The options for flues are most flexible with natural gas or LPG fired boilers. Oil fired boilers have less flexibility and the options become less flexible for larger boilers and heavier grades of oil. Finally, solid fuel boilers have the most onerous restrictions on flue terminations and the least flexibility in terms of how the flues can be arranged.

The main issue with the creation of several small unit heating systems is finding a satisfactory way of terminating the flues from the individual boilers, particularly if individual stoves are to be installed in certain locations. Importantly, gas and oil-based systems should only be installed where a clean heating system is not deemed reasonably practicable to design and install without compromising the historic character of the building.

It is most common now for domestic boilers to be wall mounted with fan assisted, balanced flues designed to be terminated locally, either vertically through roofs directly above, or through the external walls they are mounted on. Conversions that create several separate units, each with its own heating installation, will usually be damaging to the fabric and character of historic walls due to the number of penetrations required to accommodate flues and flue terminals. The positioning of an outlet from a flue is strictly controlled, which may create difficulties in the following circumstances:

a) Existing chimneys: Most traditional chimneys will comply with the minimum height requirements above the roofline, but where the height must be increased the character of the roof may be compromised. This is more likely to be a problem with flammable traditional roof coverings, such as thatch and shingles.

b) New chimneys: Unless very carefully located, the flue outlets can be very obtrusive and detract from the character of a historic roof.

It is preferable to route new metal flues within the building to reduce the possibility of condensation, but they can be mounted externally. Routing a flue within a traditional building can be difficult to achieve without considerable disruption to fabric and spaces. Alternatively, a metal flue may be fixed externally if it is insulated (twin wall is recommended) and constructed in a material suitable for combustion. However, these flues can have quite a large diameter and may be difficult to position on an external wall in a way that does not detract from the architectural or historic character of the building.

Clause 3.20.20 of the standard requires the installation of carbon monoxide (CO) gas detection devices in all residential buildings where a new or replacement fixed combustion appliance is installed. In addition, a CO detector should be installed to provide early warning to high-risk accommodation, such as a bedroom or principal habitable room, where a flue passes through these rooms. Carbon monoxide detectors in the space containing the combustion appliance should be sited between 1 metre and 3 metres from the appliance.

Additional guidance on the siting of carbon monoxide detectors, including enhanced coverage, can be found in BS EN 50292: 2023.

CO detection devices may be chosen to be as unobtrusive as possible such as being battery powered and obscurely located in key spaces to minimise risk to the historic fabric or character of the building whilst still being in a position where the buildup of CO can be detected. Compliance with this requirement should be easily achievable without significant compromise to the character of a building’s key interior spaces.

3.20.3 Issues to be considered

Typical risks in the conversion of historic/traditional buildings

1. Installation of new flues

• Both horizontal and vertical flues can be difficult to accommodate internally without disruption to historic spaces and fabric.
• External chimneys and flues can be damaging to the character of culturally significant buildings if poorly located.

2. Location of flue terminals

• Flue terminals to new chimneys must be carried above roof level to comply with Clause 3.20.17 and can be visually intrusive within a historic roofscape.
• Increasing the height of historic chimneys to meet the requirements of the standard may affect the building’s architectural or historic interest (character).
• Multiple flue terminals, for example in a conversion to separate apartments, may be damaging to both the fabric and aesthetics of a historic wall.

3.20.4 Recommendations to meet the standard

Wherever possible, a conversion to dwellings should aim to comply with the New Build Heat Standard (NBHS). This should include eliminating combustion gases from the design, although individual stoves are still permitted. Low and zero carbon systems can potentially require extensive interventions to the historic fabric of a building. These might include replacing heat emitters and installing intrusive external heat pump units which may impact on the building’s character and setting. In cases where compliance with the NBHS is not deemed to be reasonably practicable and reliance on combustion heating systems must continue, the following guidance may be considered for compliance with the standard. Clause 3.20.17 (Solid fuel appliance flue outlets) of the Domestic Technical Handbook advises that the outlet from a flue should be located externally at a safe distance from any opening, obstruction or flammable or vulnerable materials.

Natural gas or LPG fired boilers have the most flexible flue options. Many of these boilers now have the ability to run the flues for several metres horizontally, and so the boiler itself need not actually be located on an external wall. The location of the terminal must take into consideration the aesthetic impact on the building and the intervention with the fabric involved in forming the hole for the flue, as well as the normal technical requirements set out in the guide to the standards.

An alternative to the wall-mounted flue is a vertical flue arrangement. This can be individual flues for each boiler or, under certain circumstances, a combined flue arrangement for several boilers. Individual flues can be fan assisted, balanced flues working on a similar principle to the horizontal versions described above, but the range of boilers capable of utilising this system are more limited. They can also be conventional flues connected to open-flued boilers, but these are becoming less common in domestic sized boilers.

Combined flues can be designed for either conventional flues or fan-assisted balanced flue boilers. Proprietary systems exist for this and tend to handle up to ten or so boilers in each stack. Such systems have been in use throughout Europe for several years but are not commonplace in the UK. Partly because they are rare, these installations require special designs and, to be acceptable to verifiers, it is likely that detailed calculations would have to be submitted to demonstrate that the installations were satisfactory. Utilising these systems requires the boilers to be stacked, and this is often not possible in the irregular floor plans often required in historic building conversions. There are also several fire separation issues associated with vertical flues that pass through separating floors. The flues require to be housed within a fire-protected duct. These systems may be difficult to design and prove compliance with building regulations and thus may not be widely applicable.

When dealing with a larger central boiler plant for a whole building, the options for boiler flues tend to be:

i. Utilising an existing chimney by lining the flue and installing a conventional or fan-assisted conventionally flued boiler. This is often a good solution in terms of minimising the visual impact on the building, but it does rely on finding and successfully lining an existing flue, which can be problematic if the flues are structurally unsound.

ii. Installing a fan-assisted flue terminal at the top of the flue, which is a variation on (a) above. Within limits, this can help overcome problems of the flue diameter being too small to permit a normal, conventionally flued boiler to operate correctly. The modern terminal fans can be quite neat, but they are usually clearly visible on the chimneystack and the aesthetic impact of this would have to be assessed. Fan assisted terminals additionally need agreement with the boiler manufacturer and specialist design input.

iii. Installing a commercially sized, fan-assisted balanced flue boiler. These are now available in relatively large capacities but require a suitable flue termination point on an outside wall within a few metres of the boiler location. They are also exclusively natural gas or LPG fired due to the restrictions on the height for terminals associated with large oil-fired appliances. They are very useful where installing a vertical flue is not possible or would be highly disruptive.

iv. Utilising a flue dilution system is also possible for natural gas or LPG fired appliances only. In this system, the flues from conventionally flued boilers (natural draught or fan assisted) are connected into a duct and the combustion products are mixed with a large amount of fresh air, which is drawn through the duct by a fan to dilute the concentration of the combustion products to a level which can be discharged at a relatively low height. These installations require an intake and a louvered exhaust opening in a wall of the building, and the size of louvre required may be sufficiently large that its installation might have a significant impact on the character of the building. Silencers can be installed at the inlet and exhaust to reduce the noise from the fan, but, typically, these systems tend to create more noise than any of the other flue arrangements. Careful attention should therefore be paid to any relevant planning considerations.

3.20.5 Further reading

Additional guidance maybe found in the following documents:

• Domestic and Non-Domestic Technical Handbooks (2026) Sections 3.20
• British Standards Institution (2023) BS 5440-1: 2023 Chimneys, flue pipes and ventilation for gas appliances of rated input not exceeding 70 kW net (1st, 2nd, and 3rd family gases).