Quick answer
Warning signs of a commercial boiler flue fault include carbon monoxide alarms, soot or staining around boilers or flue joints, visible damage or corrosion, condensate leaking from the flue, fumes in the plant room and boilers locking out on fan or air pressure faults. Treat any of these as a safety issue: stop using affected boilers if safe, never bypass interlocks, and get a Gas Safe registered engineer.
Why flues matter
Flues carry combustion products, including carbon monoxide, safely out of the building. Fan-assisted and fan-dilution systems also rely on fans and pressure switches that stop the boilers if the flue is not working.
Warning signs
| Sign | What to do |
|---|---|
| CO alarm sounding | Evacuate, call 999 if anyone is unwell, and 0800 111 999 |
| People with headaches, dizziness or nausea near plant | Get them into fresh air and seek medical help |
| Soot, staining or scorching at boilers or flue joints | Stop affected boilers if safe; call an engineer |
| Damaged, corroded or disconnected flue | Do not run the boilers on it |
| Condensate leaking from flue joints | Report; joints may be failing |
| All boilers locked out on fan or air pressure fault | Likely dilution or flue fan fault; do not bypass |
| Unusual noise from flue fan | Report before it fails |
Never bridge or bypass a flue interlock or pressure switch. They exist to stop boilers running when combustion products are not being removed safely.
What to tell the engineer
Boiler and flue type, which boilers are affected, fault codes, what you have seen (staining, damage, alarms) and access to the flue route and roof.
Who can repair it
Flue work connected to gas appliances must be done by a Gas Safe registered business with engineers competent for commercial installations. See commercial flue repairs.
Frequently asked questions
Can a flue fault be dangerous even if the boiler is running?
Yes. A damaged or disconnected flue can let combustion products, including carbon monoxide, into the building while the boiler runs. CO alarms in plant rooms and nearby occupied areas are an important safeguard.
UK guidance and references
Primary and industry sources relevant to this subject. Always check the current edition and how it applies to your building.
- Carbon monoxide poisoning NHS
- Gas safety Health and Safety Executive
- Check an engineer or business Gas Safe Register
This guide is general information for commercial building operators. It does not replace an assessment by a competent engineer. If you smell gas or a CO alarm sounds, leave the area and call 0800 111 999; if there is a risk to life, call 999.