{"id":34928,"date":"2024-12-30T10:08:25","date_gmt":"2024-12-30T10:08:25","guid":{"rendered":"https:\/\/www.hawkins.biz\/?post_type=insight&#038;p=34928"},"modified":"2025-08-07T10:58:00","modified_gmt":"2025-08-07T09:58:00","slug":"fires-associated-with-twin-wall-flues","status":"publish","type":"insight","link":"https:\/\/www.hawkins.biz\/insight\/fires-associated-with-twin-wall-flues\/","title":{"rendered":"Fires Associated With Twin-Wall Flues"},"content":{"rendered":"\t\t<div data-elementor-type=\"wp-post\" data-elementor-id=\"34928\" class=\"elementor elementor-34928\" data-elementor-post-type=\"insight\">\n\t\t\t\t\t\t<section class=\"has_eae_slider elementor-section elementor-top-section elementor-element elementor-element-d865d26 elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-eae-slider=\"86773\" data-id=\"d865d26\" data-element_type=\"section\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"has_eae_slider elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-57003cb\" data-eae-slider=\"74282\" data-id=\"57003cb\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t<div class=\"elementor-element elementor-element-a7e512f elementor-widget elementor-widget-text-editor\" data-id=\"a7e512f\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p>In recent years, the demand for wood burning and multi-fuel stoves, as well as chimney installations has significantly increased, possibly as a result of the substantial rise in home energy and fuel costs. Consequently, this has resulted in a shortage of materials, such as stoves and chimneys, and a scarcity of qualified installers. For example, some suppliers and installers had reported lead times in excess of 18 months.<\/p><p>This shortage of skilled installers may lead to an increase in inadequate installations by inexperienced or unqualified installers, potentially leading to an increase in fires.<\/p><p>This article will focus on the installation of twin-wall flue systems, which are the most common type of chimney system used when a traditional masonry chimney is unavailable.<\/p><p>HETAS (Heating Equipment Testing and Approvals Scheme) is the certification body and industry lead for the solid fuel and biomass burning appliance industry. HETAS operate a competent person training scheme for the installation and maintenance of solid fuel appliances and chimneys. In England &amp; Wales, all stoves must be installed by HETAS registered technicians to comply with the Building Regulations; however, in Scotland and Northern Ireland, this is not a requirement. HETAS registered installers will self-certify an installation and will only be audited by HETAS on an infrequent basis.<\/p><p><span style=\"background-color: initial; color: #0a527a; font-family: soleto, sans-serif; font-size: 27px; font-weight: var( --e-global-typography-text-font-weight );\">\u00a0<\/span><\/p><p><span style=\"background-color: initial; color: #0a527a; font-family: soleto, sans-serif; font-size: 27px; font-weight: var( --e-global-typography-text-font-weight );\">Twin-Wall Flue Systems<\/span><\/p><p>A twin-wall flue is a double walled flue pipe with an insulating layer between the two walls of the pipe. The use of a twin-wall flue system allows the wood burning stove to be installed in almost any location and unlike traditional chimneys, the flue can be routed internally or externally to provide flexibility to the aesthetics of the installation. A \u2018flue\u2019 is the passage for conveying the products of combustion (the smoke and soot) to the outside air. A \u2018chimney\u2019 is a structure enclosing one or more flues. On that basis, a twin-wall flue can sometimes be referred to as a twin-wall or system chimney.<\/p><p>In my experience, the most common causes of fire relating to twin-wall flues are insufficient distance to combustible materials, poor assembly, and incompatible components.<\/p><p>Twin-wall systems for use in the United Kingdom should be designed in accordance with British Standard BS EN 1856 \u201cChimneys \u2013 Requirements for metal chimneys. System chimney products\u201d. Each component of a twin wall system provides a designation containing a series of numbers and codes (Figure\u202f1). The most important ones relating to fire are the \u2018temperature rating\u2019 and the \u2018distance to combustibles\u2019.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-0131ac1 elementor-widget elementor-widget-image\" data-id=\"0131ac1\" data-element_type=\"widget\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t<figure class=\"wp-caption\">\n\t\t\t\t\t\t\t\t\t\t<img fetchpriority=\"high\" decoding=\"async\" width=\"800\" height=\"409\" src=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fire-image.png\" class=\"attachment-large size-large wp-image-35168\" alt=\"\" srcset=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fire-image.png 830w, https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fire-image-300x153.png 300w, https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fire-image-768x392.png 768w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/>\t\t\t\t\t\t\t\t\t\t\t<figcaption class=\"widget-image-caption wp-caption-text\">Figure 1. Extract from BS EN 1856 \u2013 \u201cChimneys \u2013 Requirements for metal chimneys. \u2013 Part 1: System Chimney products\u201d. <\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-953765e elementor-widget elementor-widget-text-editor\" data-id=\"953765e\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p>Temperature ratings range from T80 to T600, which relates to the maximum operating temperature in degrees centigrade. Anything over T400 is appropriate for solid fuel burning appliances.<\/p><p>The final two numbers, specified by either G or O, provide the clear distance to combustible materials that must be maintained to the chimney\u2019s outer wall, typically 50-75\u202fmm. For comparison, a single-wall flue must have a clear distance of three times the diameter, so for a 125\u202fmm diameter flue the clear distance would be 375\u202fmm.<\/p><p>There are numerous manufacturers of twin-wall flue systems but generally, it should be assumed that different systems are not interchangeable. Regarding installation and guidance on the flue, the manufacturer\u2019s guidance is the most important, as both the HETAS guidance and building regulations, will refer to this when talking about specific systems. In my experience, the locking systems and joints from different manufacturers would not create an adequate connection and risk compromising the flue.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-91041ba elementor-widget elementor-widget-text-editor\" data-id=\"91041ba\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<h4 data-redactor-inserted-image=\"true\"><span style=\"color: #0a527a;\">Examples<\/span><\/h4>\n<p><span class=\"TextRun SCXW14580495 BCX8\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun SCXW14580495 BCX8\">In <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">one of my investigations<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">, a<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\"> wood-burning stove and twin<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">&#8211;<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">wall flue had been fitted within a workshop for heating and burning <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">wood offcuts<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">.&nbsp;<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">A<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\"> HETAS registered installer<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\"> completed the installation<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">; however, during my examination<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">,<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\"> I found a large section of <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">a <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">charred rafter to be missing <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">alongside the twin-wall flue as it passed through the roof<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">.<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">&nbsp;<\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">The <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">outer edge of the <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">flue had been installed <\/span><span class=\"NormalTextRun SCXW14580495 BCX8\">closer than the minimum distance to the rafter than recommended by the manufacturer.<\/span><\/span><span class=\"EOP SCXW14580495 BCX8\" data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">&nbsp;<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t<section class=\"has_eae_slider elementor-section elementor-top-section elementor-element elementor-element-e653e27 elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-eae-slider=\"79953\" data-id=\"e653e27\" data-element_type=\"section\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"has_eae_slider elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-cf762b0\" data-eae-slider=\"34536\" data-id=\"cf762b0\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t<div class=\"elementor-element elementor-element-ae40993 elementor-widget elementor-widget-image\" data-id=\"ae40993\" data-element_type=\"widget\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t<figure class=\"wp-caption\">\n\t\t\t\t\t\t\t\t\t\t<img decoding=\"async\" width=\"621\" height=\"775\" src=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-2.jpg\" class=\"attachment-large size-large wp-image-35169\" alt=\"\" srcset=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-2.jpg 621w, https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-2-240x300.jpg 240w\" sizes=\"(max-width: 621px) 100vw, 621px\" \/>\t\t\t\t\t\t\t\t\t\t\t<figcaption class=\"widget-image-caption wp-caption-text\">Figure 2.  An example of a twin-wall flue passing through a roof structure adjacent to a rafter.<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-07bed3f elementor-widget elementor-widget-text-editor\" data-id=\"07bed3f\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p>In another incident, a pre-existing office\/workshop within a large commercial building had a stove and twin-wall flue system installed. A few years later, a first-floor extension was constructed. The existing twin-wall flue was boxed in and packed with mineral wool insulation. CCTV showed a fire originating around the flue and penetrating through the main building roof. Opening the boxed-in flue showed heat damage to the outer wall of the flue and mineral wool insulation. In addition to this, the twin-wall flue was unsupported, as brackets had been removed and dropped into the cavity below. The lack of an air gap around the chimney prevented heat from dissipating and was not in accordance with the manufacturer\u2019s installation instructions.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-1fbbb1a elementor-widget elementor-widget-image\" data-id=\"1fbbb1a\" data-element_type=\"widget\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t<figure class=\"wp-caption\">\n\t\t\t\t\t\t\t\t\t\t<img decoding=\"async\" width=\"528\" height=\"792\" src=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-3.jpg\" class=\"attachment-large size-large wp-image-35170\" alt=\"\" srcset=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-3.jpg 528w, https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-3-200x300.jpg 200w\" sizes=\"(max-width: 528px) 100vw, 528px\" \/>\t\t\t\t\t\t\t\t\t\t\t<figcaption class=\"widget-image-caption wp-caption-text\">Figure 3. An example of a twin-wall flue wrapped in insulation before being boxed in, where a clear air gap should have been provided. <\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-a00b755 elementor-widget elementor-widget-text-editor\" data-id=\"a00b755\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p>Another example of an incorrectly constructed twin wall flue system was encountered where a fire originated at an internal support bracket supported by the floor joists. A non-standard bracket had been used rather than the proprietary manufacturer\u2019s support bracket. This resulted in a heat transfer into the joists, causing the long-term, low-temperature ignition of the structural timber. Manufacturers provide specific brackets and construction details within their literature to ensure that their products are installed in accordance with their guidance and meet the requirements of the British Standards and the Building Regulations.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-4dc7c4f elementor-widget elementor-widget-image\" data-id=\"4dc7c4f\" data-element_type=\"widget\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t<figure class=\"wp-caption\">\n\t\t\t\t\t\t\t\t\t\t<img loading=\"lazy\" decoding=\"async\" width=\"735\" height=\"493\" src=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-4.jpg\" class=\"attachment-large size-large wp-image-35171\" alt=\"\" srcset=\"https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-4.jpg 735w, https:\/\/www.hawkins.biz\/wp-content\/uploads\/2024\/12\/fig-4-300x201.jpg 300w\" sizes=\"(max-width: 735px) 100vw, 735px\" \/>\t\t\t\t\t\t\t\t\t\t\t<figcaption class=\"widget-image-caption wp-caption-text\">Figure 4. Charred floor joists, supporting a twin-wall flue utilising a fabricated bracket rather than a proprietary bracket<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-806a4c4 elementor-widget elementor-widget-text-editor\" data-id=\"806a4c4\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<h4><span style=\"color: #0a527a;\">Summary\u00a0<\/span><\/h4><p><span data-contrast=\"auto\">A twin-wall flue system is required to meet both the Building Regulations for the appropriate region and the appropriate British Standard. The easiest way for installers to achieve this is to install a system following the manufacturer\u2019s guidance, using proprietary products as recommended by the manufacturer. However, as this cannot always be the case, deviations from the manufacturer\u2019s installation guidance can leave the quality of the installation up to the skill and experience of the installer. \u00a0<\/span><span data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">\u00a0<\/span><\/p><p><span data-contrast=\"auto\">I have investigated numerous instances of fires caused by deficiencies in the installation of a twin-wall flue system, the use of non-standard components and a combination of proprietary parts from different manufacturers. \u00a0Often, the liability falls onto the installer of the system rather than any evidence of a manufacturer defect.<\/span><span data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">\u00a0<\/span><\/p><p><span data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">\u00a0<\/span><\/p><h4><span style=\"color: #0a527a;\">About the Author\u00a0<\/span><\/h4><p><a href=\"https:\/\/www.hawkins.biz\/experts\/jon-watkins\/\"><span data-contrast=\"none\">Jon Watkins<\/span><\/a><span data-contrast=\"auto\"> has investigated over 400 <\/span><a href=\"https:\/\/www.hawkins.biz\/expertise\/fire-explosions\/\"><span data-contrast=\"none\">fire and explosion <\/span><\/a><span data-contrast=\"auto\">incidents at Hawkins, leading investigations in domestic, industrial and commercial environments, ranging in size from small losses to multi-million pound complex claims. Jon has extensive experience in the legal processes in both Scotland and Northern Ireland, and the preparation of CPR Part 35 compliant reports. He has advised solicitors and Counsel and given expert evidence in the All-Scotland Sheriff Personal Injury Court, a fatal accident inquiry, and in the County Court of Northern Ireland.<\/span><span data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">\u00a0<\/span><\/p><p><span data-contrast=\"auto\">Jon is also an A2 Certificate of Competence (A2CofC) certified drone pilot and can assist clients with aerial surveys on cases relating to fires, structural failures, and personal injuries.\u202f<\/span><span data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">\u00a0<\/span><\/p><p><span data-contrast=\"auto\">If you would like Jon, or any of our other investigators to investigate an incident or failure, please <\/span><a href=\"https:\/\/www.hawkins.biz\/contact\/\"><span data-contrast=\"none\">contact us<\/span><\/a><span data-contrast=\"auto\">. <\/span><span data-ccp-props=\"{&quot;335551550&quot;:6,&quot;335551620&quot;:6}\">\u00a0<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t<section class=\"has_eae_slider elementor-section elementor-top-section elementor-element elementor-element-a3abccb elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-eae-slider=\"43151\" data-id=\"a3abccb\" data-element_type=\"section\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"has_eae_slider elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-b20054d\" data-eae-slider=\"65454\" data-id=\"b20054d\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap\">\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"<p>This article will focus on the installation of twin-wall flue systems and the dangers they can pose if not installed to the safety standard set out in the Building Regulations. <\/p>\n","protected":false},"featured_media":34937,"parent":0,"template":"","insight_category":[],"experties":[66],"class_list":["post-34928","insight","type-insight","status-publish","has-post-thumbnail","hentry","experties-fire-explosions"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v25.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Twin-Wall Flue Fires: Causes &amp; Prevention<\/title>\n<meta name=\"description\" content=\"Learn how improper installation of twin-wall flues can lead to fires, and how forensic analysis identifies faults and ensures compliance.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.hawkins.biz\/insight\/fires-associated-with-twin-wall-flues\/\" 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