Flue in Voids (TB 008) for Gas Engineers - Viva Training Centre

In gas safety, the management and understanding of flues in voids emerge as a critical aspect, often overshadowed by more immediate concerns. However, the integrity of these concealed pathways, through which hazardous gases are expelled from domestic and commercial properties, is paramount to ensuring the safety and well-being of occupants. Flues hidden within the fabric of buildings—in wall cavities, floor spaces, or above ceilings—pose significant challenges for maintenance, inspection, and compliance with evolving gas safety regulations.

Enter Technical Bulletin 008 (Edition 3), a document developed by the Industry Flues in Voids Working Group, explicitly aimed at gas engineers. This bulletin provides a guide detailing best practices, legal obligations, and safety measures for dealing with existing concealed room-sealed fanned-draught boiler chimney/flue systems in domestic premises. Its significance cannot be overstated; TB 008 (Edition 3) provides gas engineers with the guidance needed to navigate the complexities of flues in voids and ensures that they comply with the Gas Safety (Installation and Use) Regulations 1998.

The focus of this guide is multifaceted, addressing the immediate need for safe and effective flue systems while also acknowledging the long-term implications of their maintenance and inspection. For gas engineers, adherence to the recommendations and guidelines outlined in TB 008 (Edition 3) is a step towards guaranteeing the safety of gas installations, mitigating the risks of carbon monoxide poisoning, and upholding the integrity of their profession.


Flues in Voids

A flue in a void is a duct for gases from combustion appliances, such as boilers, to exit a building from areas not visible or directly accessible, like wall cavities, ceiling voids, or spaces beneath floors. These concealed flues are particularly prevalent in modern residential and commercial buildings, designed to maximise living space while keeping the mechanical components of the building out of sight. The most affected properties include apartment complexes, multi-story residential buildings, and commercial properties built or renovated within the last two decades. These structures often rely on room-sealed fanned-draught systems, where the flue is an integral part of the building’s construction, making it challenging to access for inspection, maintenance, or repair.


Correctly managing flues in voids is not just a matter of regulatory compliance; it’s a critical safety issue. The Gas Safety (Installation and Use) Regulations 1998 set clear guidelines and obligations for installing, inspecting, and maintaining gas appliances and their flues to prevent the risk of carbon monoxide (CO) poisoning, gas leaks, and fires. Compliance with these regulations, and by extension, adherence to the guidance provided in Technical Bulletin 008 (Edition 3), is essential for several reasons:

The primary concern is the safety of building occupants. Improperly installed or maintained flues can lead to the leakage of carbon monoxide, a deadly gas that is odourless and colourless, making its detection without proper alarms nearly impossible. Ensuring flues in voids are correctly managed mitigates this risk significantly.

Legal Obligations:
Gas engineers and property owners have legal responsibilities under the Gas Safety Regulations to ensure that gas appliances and flues are safe and fit for purpose. Non-compliance can result in legal action, including fines and, in severe cases, imprisonment.

Professional Integrity:
For gas engineers, compliance with the regulations and guidance documents like TB 008 (Edition 3) reflects their professional integrity and commitment to safety. Following these guidelines indicates their dedication to best practices in the gas safety field.

Preventative Maintenance:
As the bulletin recommends, regular inspection and maintenance of flues in voids can prevent significant hazards and costly repairs. Compliance ensures that potential issues are identified and addressed before they become dangerous or disruptive.


Gas Safety (Installation and Use) Regulations 1998

The Gas Safety (Installation and Use) Regulations 1998, particularly Regulation 26(9), play a pivotal role in the management and safety of flues in voids. This regulation requires that any work on gas appliances, including boilers, examine the flue’s effectiveness and proper function. This ensures that the flue correctly removes harmful combustion products from the appliance to the outside atmosphere, thus preventing the accumulation of dangerous gases within the premises.

For flues in voids, those concealed within the fabric of the building and not directly visible or accessible—the application of Regulation 26(9) presents unique challenges. It mandates that gas engineers must have the means to inspect the entire length of these flues to confirm their integrity and correct operation. This requirement underlines the need for inspection hatches or other access points to be available or installed, enabling engineers to carry out their legal duties to inspect flues thoroughly.

Role of Gas Engineers

The responsibilities placed on registered gas engineers regarding flue safety and compliance are comprehensive and multifaceted. As the professionals tasked with the implementation of the Gas Safety Regulations, their role encompasses several duties:

Inspection and Testing:
Engineers must inspect and test flues for effectiveness when working on a gas appliance. This includes ensuring that flues are appropriately sealed, supported, and terminated correctly to safely convey combustion products out of the building.

Identification and Reporting of Issues:
Should any issues with the flue’s integrity, installation, or operation be discovered, the engineer must report these findings to the property owner or responsible party. This may include identifying flues that cannot be inspected due to lack of access and recommending the installation of inspection hatches.

Advisory Role:
Beyond inspection and repair, gas engineers advise property owners on the importance of flue safety, the potential risks associated with flue faults, and the legal requirements for gas safety compliance. They may also recommend the installation of carbon monoxide detectors as an additional safety measure.

Compliance and Documentation:
Engineers must ensure that all work complies with the current regulations and standards. This includes maintaining detailed records of inspections, tests, and any work carried out on gas appliances and flues. These records are crucial for demonstrating compliance with safety regulations and for future reference during subsequent inspections or maintenance work.

Education and Continuous Learning:
Keeping up-to-date with the latest regulations, guidance, and best practices, such as those detailed in Technical Bulletin 008 (Edition 3), is a continuing responsibility for gas engineers. Ongoing education ensures that engineers are equipped to manage the evolving challenges of gas safety and flue management.


Technical Bulletin 008 (Edition 3) Overview

Technical Bulletin 008 (Edition 3), developed by the Industry Flues in Voids Working Group, represents a significant advancement in gas safety. This document emerged in response to identified risks associated with concealed flues in modern construction practices, where flues are often hidden within the fabric of the building, making inspection and maintenance challenging. The primary purpose of TB 008 (Edition 3) is to provide comprehensive guidance to gas engineers on how to approach these challenges effectively, ensuring compliance with the Gas Safety (Installation and Use) Regulations 1998.

The bulletin was developed with input from industry experts, regulatory bodies, and safety organisations, reflecting a consensus on best practices for managing the unique risks posed by flues in voids. Its objectives are multifaceted:

  • To outline the responsibilities of gas engineers in inspecting and maintaining concealed flues.
  • To provide practical solutions for ensuring the integrity and safety of these flue systems.
  • To promote awareness of the potential dangers of improper flue management.
  • To facilitate compliance with legal and regulatory requirements, thus enhancing overall gas safety in domestic and commercial buildings.

Guidance Provided

Technical Bulletin 008 (Edition 3) offers detailed guidance on dealing with flues in voids, targeting registered gas engineers tasked with inspecting, maintaining, and certifying gas appliances and their flues. Key highlights include:

Inspection Access:
The bulletin stresses the importance of proper access to inspect concealed flues thoroughly. It recommends the installation of inspection hatches in strategic locations to allow for a thorough visual examination of the flue’s entire length.

Carbon Monoxide Alarms:
TB 008 underscores the critical role of carbon monoxide (CO) alarms in properties with concealed flues. It advises on the optimal placement of CO alarms to ensure effective monitoring of potential leaks, enhancing occupant safety.

Identification of Defects:
The document guides on identifying common installation defects that may compromise the safety and integrity of flue systems, such as inadequate supports, improper sealing, or incorrect flue materials. It also outlines remedial actions to address these defects.

Risk Classification:
For flues that cannot be inspected due to lack of access or other constraints, TB 008 offers a framework for classifying the associated risks. It details procedures for categorising installations as ‘At Risk’ or ‘Immediately Dangerous,’ including steps for notifying property owners and taking appropriate safety measures.

Alternative Solutions:
Recognising that, in some cases, installing inspection hatches may not be feasible, the bulletin explores alternative solutions, such as using CO void monitoring safety shut-off systems (COSSVM). These systems provide an added layer of protection by detecting CO within the void and automatically shutting off the gas supply to the appliance.

Documentation and Reporting:
The bulletin emphasises the importance of thorough documentation and reporting of inspections, assessments, and any corrective actions taken. This ensures a traceable record of compliance and safety measures implemented.

Technical Bulletin 008 (Edition 3) is an essential resource for gas engineers, offering a roadmap for navigating the complexities associated with concealed flues.


Best Practices for Handling Flues in Voids

Managing flues in voids requires adherence to best practices to ensure the safety and compliance of gas installations. Technical Bulletin 008 (Edition 3) outlines critical measures that gas engineers should follow, focusing on inspection access, carbon monoxide detection, and the maintenance of flue systems.

Inspection Hatches

Inspection hatches provide access to concealed flues, allowing gas engineers to perform necessary inspections and maintenance. Without these hatches, it’s nearly impossible to assess the condition of flues hidden within walls, ceilings, or floors, posing significant safety risks.

TB 008 provides detailed specifications for the installation of inspection hatches. Key recommendations include:

Size and Location:
Hatches should be large enough to allow a thorough visual inspection of the flue system. A minimum size of 300mm x 300mm is often recommended, though the size may vary based on the flue configuration and the need for access to specific points along the flue route.

Number and Placement:
The number and placement of inspection hatches should be determined based on the length of the flue and the points where access is most critical for inspection. Hatches should be strategically located to cover the flue system, ensuring no section remains uninspected.

Sealing and Safety:
Hatches must be adequately sealed and insulated to prevent heat loss and the ingress of harmful gases into living spaces. They should also be designed to maintain the building’s fire safety and structural integrity.

Carbon Monoxide (CO) Alarms


  • CO alarms should be placed in areas where they can effectively detect the presence of carbon monoxide, providing an early warning to occupants. Ideal locations include sleeping areas and living spaces near the flue route, ensuring that alarms can detect CO leaks from any point along the concealed flue.
  • Alarms should be installed according to the manufacturer’s instructions, typically at breathing height and away from direct sources of heat or humidity that could affect their operation.


  • Regular testing and maintenance of CO alarms are crucial to ensure their functionality. This includes following the manufacturer’s recommendations for testing the alarm function, typically monthly, and replacing batteries annually or as needed.
  • Alarms should be replaced according to the manufacturer’s lifespan recommendation, usually every 5-10 years, to ensure reliability.

Ongoing maintenance of gas appliances and their flues ensures their safe operation. Regular service by registered engineers helps identify and rectify potential issues before they become hazardous.

Recommended Practices:

  • A qualified gas engineer should inspect gas appliances and flues annually. These inspections should include a thorough check of the appliance, the integrity and effectiveness of the flue, and adequate ventilation.
  • Flues should be cleaned regularly to remove any blockages or build-up of soot and debris, which could impede the safe expulsion of combustion gases.
  • Engineers should maintain detailed records of all inspections, maintenance, and repairs performed, providing a history of the appliance and flue system’s condition and compliance.


Addressing Installation Defects

Proper installation and maintenance of flues are critical to ensuring the safe operation of gas appliances. However, defects can occur over time or due to improper installation. Identifying and rectifying these defects is essential in maintaining the integrity of flue systems, especially those concealed within voids.

Typical Installation Defects and Risks

Several typical installation defects could compromise the integrity and safety of flue systems. These include:

Inadequate Support:
Flues that lack proper support can sag or disconnect, leading to leaks of combustion products into the building.

Improper Sealing:
Joints and connections within the flue system that are not adequately sealed can allow harmful gases to escape.

Incorrect Slope:
Flues designed to drain condensate that does not have the correct slope can lead to water pooling, which may corrode the flue or block the flow of gases.

Use of Inappropriate Materials:
Using materials that are not resistant to the temperatures and chemicals in flue gases can result in deterioration and failure of the flue system.

Blockages within the flue, caused by debris, nesting animals, or construction materials, can prevent the proper venting of combustion gases.

These defects pose immediate risks of carbon monoxide poisoning and fire and can lead to long-term issues such as appliance failure and structural damage.

Remedial Actions

When identifying any defects in a flue system, especially within concealed voids, the following remedial actions are recommended to ensure the safety and compliance of the installation:

Support Correction:
Install additional supports or brackets to ensure the flue is correctly supported along its length, preventing sagging and disconnection.

Sealing of Joints:
Use appropriate sealants or gaskets to repair any leaks at joints or connections within the flue system. Ensure that the materials used suit the temperature and composition of the flue gases.

Adjustment of Slope:
Reconfigure the slope of the flue to ensure condensate can drain back to the appliance or a suitable drainage point without pooling.

Replacement of Materials:
Replace any sections of the flue system made from inappropriate materials with those specified by the appliance manufacturer or relevant standards, ensuring resistance to flue gas temperatures and chemistry.

Clearance of Obstructions:
Remove any blockages within the flue system, checking both visually and using appropriate tools. Regular inspections should be conducted to prevent future obstructions, especially in areas prone to nesting or debris accumulation.

Professional judgment and consultation with building owners and authorities may be necessary for defects that cannot be easily rectified, such as those requiring significant alteration of the building structure or those in highly difficult-to-access areas. This might include system redesign, additional inspection access points, or replacing the entire flue system to meet current safety standards.


Alternative Solutions and Considerations

In the complex landscape of gas safety, particularly regarding flues in voids, engineers and property owners sometimes face situations where traditional solutions, such as installing inspection hatches, may not be feasible or sufficient. In such cases, alternative solutions and a thorough understanding of regulatory compliance and building regulations become crucial.

CO Void Monitoring Safety Shut-Off Systems (COSSVM)

The use of Carbon Monoxide Void Monitoring Safety Shut-Off Systems (COSSVM) offers an innovative alternative to inspection hatches, especially in scenarios where installing hatches is impractical or could compromise the structural integrity or aesthetic of the building. These systems are designed to monitor the void space for carbon monoxide continuously, a clear indication of a malfunctioning flue system.

Upon detecting elevated levels of CO, the system automatically shuts off the gas supply to the appliance, significantly reducing the risk of carbon monoxide poisoning. This immediate response provides a critical safety net, mitigating potential hazards before they can escalate into serious health threats.

Considerations for COSSVM Installation:

Sensitivity and Reliability:
Ensure the system chosen meets industry standards for CO detection sensitivity and reliability.

Correct placement of sensors is essential for effective monitoring. Sensors should be located in areas most likely to accumulate CO if there is a leak.

Regular maintenance checks are required to ensure the system remains operational and practical.

While COSSVM systems provide an added layer of safety, they should not be considered a replacement for regular flue inspections and maintenance. They are best used in a comprehensive safety strategy, complementing other measures like CO alarms and regular gas appliance servicing.

Regulatory Compliance and Building Regulations

Technical Bulletin 008 (Edition 3) offers guidance on best practices for managing flues in voids and ensures that the recommended approaches align with existing building regulations and standards. Understanding how TB 008 integrates with these regulations is essential for gas engineers to ensure compliance with new installations and the maintenance of existing systems.

Alignment with Building Regulations:

New Installations:
TB 008 guidance emphasises the importance of planning for flue inspection and access from the outset for new or replacement installations. This forward-thinking approach ensures that new systems are installed in compliance with current building regulations, which often require direct access for inspection and maintenance.

Existing Installations:
The bulletin provides a framework for bringing these systems up to current safety standards, even when direct access for flue inspection is limited. By recommending the installation of inspection hatches where possible and detailing the use of alternative safety measures like COSSVM systems, TB 008 helps bridge the gap between older installations and modern safety requirements.

Compliance Considerations:

Documentation: Keeping detailed records of compliance efforts, including installing safety systems and any modifications made to accommodate inspection and maintenance, is crucial.

Regulations: Gas engineers must also be aware of local or regional regulations that may impose additional requirements or standards beyond those covered in TB 008.

Alternative solutions like COSSVM systems offer valuable safety enhancements. Still, adherence to TB 008 and a comprehensive understanding of regulatory compliance and building regulations remain fundamental to ensuring the safety and integrity of flues in voids.


Practical Application of TB 008

The real-world application of Technical Bulletin 008 (Edition 3) provides valuable insights into the challenges and solutions of managing flues in voids. Through case studies and a Q&A section, we can explore how the guidance has been successfully implemented and address common questions from gas engineers.

Case Studies and Examples

Case Study 1: Multi-Unit Residential Building

In a large apartment complex built in the early 2000s, several units were found to have concealed flues in voids without any inspection access. Following TB 008 guidance, the property management collaborated with gas safety engineers to install inspection hatches along the flue routes at strategic locations. This allowed for thorough inspections and led to the discovery and remediation of several minor defects that, if left unchecked, could have posed significant risks to residents.

Case Study 2: Retrofitting COSSVM in a Historic Building

A historic building converted into residential apartments presented unique challenges due to the inability to modify the structure extensively. In this scenario, engineers opted to install a CO Void Monitoring Safety Shut-Off System as recommended by TB 008. This system provided an effective alternative to inspection hatches, ensuring resident safety without compromising the building’s architectural integrity. The COSSVM system’s successful implementation underscored the flexibility and adaptability of TB 008’s guidance in addressing flue safety in complex environments.

Q&A Section

Q1: How can I determine the best locations for installing inspection hatches?

The ideal locations for inspection hatches allow visual access to the entire length of the concealed flue. Key points include bends, joints, and areas where the flue passes through different building levels. TB 008 recommends consulting the flue installation plans and conducting a thorough risk assessment to identify critical access points.

Q2: Are there any specific requirements for the CO alarms recommended by TB 008?

TB 008 aligns with industry standards, recommending that CO alarms meet BS EN 50291 standards and be installed according to the manufacturer’s guidelines. Placement should ensure adequate detection of CO, such as in sleeping areas and proximity to the gas appliance and flue system.

Q3: What should I do if it’s not feasible to install inspection hatches due to building restrictions?

In cases where structural limitations prevent the installation of inspection hatches, TB 008 suggests considering alternative safety measures, such as COSSVM systems. These systems offer a way to monitor for CO within voids and automatically shut off the gas supply in case of detection, providing a critical safety mechanism without direct flue access.

Q4: How frequently should concealed flues be inspected?

TB 008 recommends that concealed flues be inspected annually as part of the gas appliance’s regular service and maintenance schedule. However, the frequency may increase based on the appliance’s age, usage patterns, and any wear or damage observed during inspections.


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