This is the third in our mini-series inspired by our Halloween competition.

Carbon Monoxide Deaths in UK Homes

Carbon monoxide (CO) is often referred to as the “silent killer.” It is a colourless, odourless and tasteless gas that can build up in homes without warning. Despite being preventable, carbon monoxide poisoning still causes deaths and serious injuries in the UK each year. Over 60 people die each year in the UK from carbon monoxide poisoning alongside around 4,000 emergency department treatments and 200 hospitalisations. Approximately 45% of these occur in homes, which equates to around 27–30 preventable deaths annually in domestic properties. Yet many designers apparently remain unaware that they should be designing and ensuring combustion to meet the requirements of Building Regulations, and it is rare to see any detailed design in most domestic applications.

The Problem

Carbon monoxide (CO) is produced whenever fuel is burned – gas boilers, fires, cookers, and heating appliances are the primary sources. In properly functioning systems, CO is safely discharged outside via flues. When flues fail, become blocked, or are poorly designed, CO accumulates in occupied spaces, poisoning occupants within hours.

Central heating boilers account for just over a quarter of fatalities, with gas combustion causing the greatest percentage of deaths overall, while solid fuel systems show disproportionately high death rates per user. Alarmingly, 36% of UK households lack a CO alarm – a basic protective measure that should be standard.

What is Approved Document J?

Approved Document J came into force to establish performance requirements for combustion appliances and associated flue systems. It addresses four critical areas: air supply for combustion, discharge of combustion products, carbon monoxide warning systems, and inspection and maintenance access.

The Architectural Role

Combustion safety depends heavily on building design and ease of maintenance – and Architects designs can significantly influence whether flue systems function safely.

Adequate Ventilation

Combustion appliances must have sufficient air supply for safe operation. Without adequate oxygen, incomplete combustion occurs, producing carbon monoxide. Modern dwellings built to high energy efficiency standards present unique challenges when combustion appliances are used.

Highly airtight envelopes can create:

• Negative internal pressure from extract ventilation
• Reduced infiltration air supply
• Increased potential for flue back drafting

These effects can interfere with the safe operation of combustion appliances. For example, log burners remain a popular focal point in architectural designs but can present significant life safety issues in a truly airtight building with mechanical extraction. In such cases the stove installation must feature a direct external air supply (DEAS) independent of the room air, and the stove should have an effective door-seal to make it and its flue virtually 100% room-sealed. This means none of its combustion air can be drawn from the room, and conversely none of the combustion gases can escape.

Equally homes with traditional open fireplaces are often not suitable for many MVHR systems as the air bricks that supply the combustion air to the fire undermine the airtightness required for MVHR efficiency. Contractors and homeowners can be tempted to close off the traditional air bricks to ensure the MVHR system is working efficiently, unaware of the danger they are causing if they then use the open fireplace.

Flue Design and Routing

Requirements of Part J include:

• Correct sizing of flues
• Continuous passage to outside air
• Adequate termination above roof level
• Prevention of leakage back into the dwelling

Poor architectural coordination creates hidden risks:

• Long horizontal flue runs across the dwelling allow condensation to accumulate, leading to corrosion and leakage
• Multiple bends around structural elements increase resistance and can trap combustion gases
• Terminations beneath balconies or soffits allow exhaust recirculation into occupied spaces
• Flues routed through inaccessible ceiling voids prevent inspection and maintenance

Safe design principles are straightforward: keep flues as short as possible, specify vertical rather than horizontal flues, and direct external discharge and termination positions to prevent gas recirculation. The cost of including these design features is negligible compared to the risk of CO poisoning.

Inspection Access

Approved Document J explicitly requires that concealed flues remain inspectable, particularly at joints and connections. Yet we commonly see flues disguised and boxed in behind plasterboard without access panels, making it impossible for engineers to verify installation, detect corrosion, or confirm safe operation during commissioning.

Proper design includes removable panels, service cupboards, or dedicated inspection hatches. These simple provisions allow future servicing and reduce the risk of undetected leaks. Access panels in a ceiling may not be pretty, but they could save a life!

Alarm Provision and Maintenance

Clause 1.24 of Approved Document J (2022) requires that where a new or replacement fixed flued combustion appliance is installed in a dwelling, a carbon monoxide alarm complying with BS EN 50291-1:2018 should be provided. Alarms must include an integral sounder and battery designed to last the working life of the device.

The alarm should also

• Be installed in the same room as the appliance.
• Be located in accordance with the manufacturer’s instructions.
• Be positioned where occupants can hear it clearly.

However, despite this requirement and widespread awareness of CO risks, 36% of UK households remain without a CO alarm. Moreover, alarms are a last-resort protection, not a substitute for safe appliance installation and maintenance.

Building Control and Professional Responsibility

All too often Combustion Appliance design is left as a contractor design portion, without establishing if the contractor is competent to design. Remember that as a Principal Designer you remain liable for the work of other designers and simply leaving it to the Contractor can open you to a claim. To ease your way through the Building Control Process and avoid liability please ensure:

• Design Review: Specify appliance locations, flue routes, and termination points as part of your submission. Scrutinise and challenge contractors proposed designs that create long horizontal runs, multiple bends, or inaccessible concealed flues.
• Commissioning Verification: Ensure that upon completion, the contractor is required to verify flue systems have been properly inspected, tested, and certified by qualified engineers.
• Occupier Education: Make clear to building owners and occupants the maintenance requirements for combustion appliances.

The Design Question: Why Combustion at All?

Given the risks involved it raises a broader question: should combustion appliances be specified at all?

Modern low-carbon heating systems – air source heat pumps, ground source heat pumps, heat networks – eliminate combustion entirely. They remove CO production risk, eliminate flue systems, simplify ventilation coordination, and reduce maintenance burden. As building regulations increasingly prioritise electrified heating and net-zero carbon targets, specifying gas combustion in new homes introduces unnecessary safety complexity alongside carbon performance concerns. Why continue to take that risk?