BoilerBuddy · safety guide
Carbon monoxide safety — what to look out for, and what to do next.
Carbon monoxide is invisible and has no smell. This page lists the NHS symptoms, what a yellow or orange boiler flame can mean, and the steps to take if you suspect a leak: stop using the appliance if you can, ventilate on the way out and go outside. It also explains which service to call for a suspected leaking gas appliance or possible symptoms.
Suspect a leaking gas appliance? Leave and call 0800 111 999
Stop using the appliance if you can, open doors and windows as you leave, and go outside. If a gas appliance may be leaking carbon monoxide, call the National Gas Emergency Service on 0800 111 999. For possible CO symptoms, call NHS 111. For the NHS-listed severe symptoms, call 999 for an ambulance or go to A&E; do not drive. Call 999 for fire and rescue only if there is an actual fire.
Why this matters
Carbon monoxide is invisible, odourless, and worth taking seriously.
Carbon monoxide is invisible and odourless. If your CO alarm is sounding or you suspect a leak, stop using the appliance if you can, open doors and windows on the way out, and go outside. If you suspect a gas appliance is leaking carbon monoxide, call the National Gas Emergency Service; if you have possible symptoms, call NHS 111. This page is information before a registered engineer takes over.
Symptoms to look out for
Nine symptoms listed by the NHS for carbon monoxide poisoning.
The NHS lists these symptoms of carbon monoxide poisoning. Symptoms may come and go. They may get worse when you spend time in an affected room or building and get better when you leave or go outside.
- 1
a headache
- 2
dizziness
- 3
feeling sick or being sick
- 4
feeling weak
- 5
tiredness and confusion
- 6
chest and muscle pain
- 7
shortness of breath
- 8
your face turning hot and red (flushing) – but redness may be harder to see on brown and black skin
- 9
loss of vision
This page is information, not a diagnosis. For possible CO symptoms, call NHS 111. If you think someone has been exposed and is finding it hard to breathe, has suddenly become confused, has lost consciousness, is feeling weak or has chest or muscle pain, call 999 for an ambulance or go to A&E. Do not drive to A&E; ask someone to drive or call 999 for an ambulance. Call 999 for fire and rescue only if there is an actual fire.
What a yellow or orange flame means
A healthy boiler flame is blue. Yellow, orange or sooty is a sign to stop using it.
In a working boiler the burner flame is crisp, even and blue, with a steady shape. A yellow, orange or ‘lazy’ flame — or any soot or dark staining around the burner window — is a sign that combustion is incomplete. The same chemistry that produces soot is the chemistry that can produce carbon monoxide inside the appliance. BoilerBuddy is not a diagnostic tool: the safe move is to stop using that boiler and have a Gas Safe registered engineer look at it before it goes back on.
Don’t try to ‘fix’ a yellow flame yourself. UK Gas Safety (Installation and Use) Regulations reserve appliance work on a gas appliance for a Gas Safe registered engineer, and the safe move when the flame is wrong is the same as the safe move when an alarm goes off — stop, ventilate, leave, call.
What to do if you suspect a leak
STOP using the appliance if you can, VENTILATE by opening doors and windows on the way out, and LEAVE the property for fresh air. If a gas appliance may be leaking carbon monoxide, call the National Gas Emergency Service on 0800 111 999. For possible symptoms call NHS 111; for the NHS-listed severe symptoms call 999 for an ambulance or go to A&E, and do not drive. Call 999 for fire and rescue only if there is an actual fire.
- 1
STOP — switch off the boiler at the controls
Turn the boiler off at the room thermostat, the programmer and the boiler itself. Do not reset a sounding CO alarm to keep using the heating. Stop using any other gas appliance in the property until a registered engineer clears them — a gas hob, oven or fire could be contributing to the problem.
- 2
VENTILATE — open doors and windows on the way out
Open doors and windows as you leave the property if it costs you no extra delay. The aim is to dilute any CO that has built up. Don’t stop to fiddle with appliances or to switch things off beyond what is already done — getting out cleanly is more important.
- 3
LEAVE — get everyone (and pets) out
Take everyone outside, including pets, and do not go back into the affected building until you have medical advice. For possible CO symptoms, call NHS 111. If anyone has difficulty breathing, sudden confusion, loss of consciousness, weakness or chest or muscle pain, call 999 for an ambulance or go to A&E; do not drive.
- 4
CALL — use the right service for the situation
If you suspect a gas appliance is leaking carbon monoxide, call the National Gas Emergency Service on 0800 111 999 from outside. If you have possible CO symptoms, call NHS 111. For the NHS-listed severe symptoms, call 999 for an ambulance or go to A&E; do not drive to A&E, ask someone to drive or call an ambulance. Call 999 for fire and rescue only if there is an actual fire.
Source: National Gas emergency guidance
If nobody is in immediate danger
Book a Gas Safe engineer to check the boiler.
A single CO alarm sound, or a one-off yellow flame sighting, doesn’t always mean a present-day leak — but it does mean a registered engineer should check the appliance before normal use resumes. Use the directory below or the official Gas Safe Register to book a visit.
FAQ
The five questions this page raises.
Related guides
If you’d rather start from a different angle.
Safety
Emergency safety guide
Three STOP / LEAVE / CALL scenarios — gas smell, CO alarm sounding, repeated lockouts — with tap-to-call numbers for National Gas and 999.
emergency safety guide →Worcester Bosch
Worcester Bosch fault codes
Type a code off the boiler display — we’ll tell you what it means, what to safely try, and the line that means ‘call an engineer’.
worcester bosch fault codes →Vaillant
Vaillant fault codes
F.22, F.28, F.29, F.36, F.62 and the rest — same plain-English walk-through, same clear handoff point.
vaillant fault codes →Ideal
Ideal fault codes
The plain-English walk-through of the Ideal fault codes that surface most often, with the same safe-at-home → call-an-engineer line.
ideal fault codes →Baxi
Baxi fault codes
The Baxi fault codes that surface most often, translated into plain English and paired with the safe-at-home → engineer line.
baxi fault codes →Related symptom
Boiler keeps losing pressure
If pressure is also dropping on the same appliance, the same engineer visit usually covers both checks — the system-side sibling page.
boiler keeps losing pressure →Related symptom
Boiler making banging or kettling noises
Kettling on its own is usually scale, not CO, but loud kettling plus any symptom on this page is reason to escalate. The third symptom guide.
boiler making banging or kettling noises →