Carbon monoxide (CO) is an invisible, odourless gas that kills. Every year, people are seriously injured or die from CO poisoning on boats in the UK. Narrowboats are particularly high risk because of the combination of enclosed living spaces, solid fuel stoves, gas appliances, and engine exhaust — all in a small, confined area.
This isn't a scare piece — it's a practical guide to understanding the risks and making sure your boat is safe.
Why Narrowboats Are High Risk
Several factors make narrowboats especially vulnerable to CO build-up:
- Enclosed space: A narrowboat is a sealed steel box just under 7 feet wide. There's very little air volume compared to a house, so even a small CO leak can quickly reach dangerous concentrations.
- Solid fuel stoves: Wood and coal burners are the most common heating on liveaboard boats. If the flue is blocked, cracked, or poorly sealed, or if the stove itself is damaged, CO can leak into the cabin instead of going up the chimney.
- Gas appliances: Cookers, water heaters, and gas fridges all produce CO as a byproduct of combustion. If they're poorly maintained, incorrectly installed, or the ventilation is inadequate, CO levels can rise.
- Engine exhaust: Diesel engines produce CO. If exhaust fumes can get into the cabin — through leaking exhaust joints, a cracked exhaust manifold, or when the engine is running at tick-over with the stern doors open — it's dangerous.
- Reduced ventilation in winter: This is when the risk is highest. Boaters close up hatches and vents to stay warm, which reduces the fresh air supply that CO-producing appliances need and traps any CO that's produced.
CO Alarm Requirements
The Boat Safety Scheme (BSS) has clear requirements for carbon monoxide detection:
- At least one CO alarm is mandatory on any boat with a solid fuel appliance (stove, back boiler, etc.). This has been a BSS requirement since 2019.
- The alarm must conform to BS EN 50291. This is the European standard for CO detection. Check the label on your alarm — it should state this standard clearly.
- Placement: The alarm should be mounted at head height in the same space as the appliance, ideally 1-3 metres from the stove. Don't put it directly above the stove, in a dead air corner, or near an opening window or vent.
- Battery or mains powered: Either is fine, but if it's battery powered, check it regularly and replace batteries as recommended by the manufacturer.
- Replace alarms on schedule. Most CO alarms have a lifespan of 5-7 years. There will be a replacement date printed on the alarm. Don't ignore it — the sensor degrades over time and becomes unreliable.
While the BSS only mandates an alarm near solid fuel appliances, it's strongly recommended to have CO alarms in any space with a combustion appliance, and near sleeping areas.
Ventilation Rules
Proper ventilation is your first line of defence against CO. The BSS has specific ventilation requirements:
- Fixed ventilation: Every space with a combustion appliance must have permanent, fixed ventilation that cannot be closed. This typically means mushroom vents or similar fittings in the cabin roof or sides.
- Size matters: The required ventilation area depends on the appliance's heat output. A solid fuel stove typically needs a minimum of 100cm² of free air opening. Your BSS examiner will check this.
- Never block vents: This is the single most dangerous thing boaters do. Blocking vents to keep warm in winter is a CO risk. If you're cold, add insulation, wear more layers, or improve your heating — but never block your ventilation.
- Flu draught: Solid fuel stoves need good draught to pull exhaust gases up the chimney. If the flue is too short, blocked, or the chimney cowl is wrong, the stove can "back-puff" CO into the cabin.
Signs of Carbon Monoxide Poisoning
CO is called the "silent killer" because you can't see, smell, or taste it. The symptoms of CO poisoning can be easily mistaken for flu, tiredness, or a hangover:
- Headache (the most common early symptom)
- Dizziness and nausea
- Tiredness and confusion
- Stomach pain
- Shortness of breath
- Loss of consciousness (at high concentrations)
Key warning sign: If your symptoms improve when you leave the boat and return when you come back aboard, suspect CO immediately.
Long-term exposure to low levels of CO can cause lasting neurological damage, even if you never experience acute poisoning symptoms. This is why alarms and ventilation are so important.
Maintenance to Prevent CO
Regular maintenance is essential for preventing CO build-up:
Solid Fuel Stoves
- Have the flue swept and inspected at least once a year, ideally before the heating season starts in autumn.
- Check the flue collar seal where the flue pipe meets the stove. Any gaps here will leak CO directly into the cabin.
- Inspect the stove body for cracks, especially around welds and the door seal.
- Make sure the chimney cowl is in good condition and allows proper draught.
- Burn appropriate fuel — don't burn treated wood, painted wood, or plastics, which produce more CO and toxic fumes.
Gas Appliances
- Have gas appliances serviced annually by a Gas Safe registered engineer (ideally one with marine experience).
- Check the flame colour. A gas flame should be blue. Yellow or orange flames indicate incomplete combustion, which produces more CO.
- Keep burners clean and unblocked.
- Ensure proper ventilation around all gas appliances.
Engine
- Check the exhaust system regularly for leaks, corrosion, or loose joints.
- Make sure the exhaust exits well away from any cabin openings.
- Don't run the engine at tick-over for long periods with the stern doors open — exhaust can be drawn back into the boat.
What to Do if Your CO Alarm Goes Off
If your carbon monoxide alarm sounds, act immediately:
- Stop and think: Don't panic, but do act quickly.
- Open doors and windows: Get fresh air flowing through the boat immediately.
- Turn off all appliances: Extinguish the stove (close the air vents to let it die), turn off gas appliances, turn off the engine.
- Get everyone off the boat: Move to open air. Account for everyone, including pets.
- Call for help: If anyone feels unwell, call 999 immediately. CO poisoning is a medical emergency.
- Don't go back inside until the source has been identified and fixed.
- Get the appliance checked: Have a qualified engineer inspect the suspected source before using it again.
Even if the alarm stops and everyone seems fine, don't just reset it and carry on. Find out what triggered it.
BSS Requirements for CO Safety
The Boat Safety Scheme checks several CO-related items during the four-yearly examination:
- CO alarm present and in date (for boats with solid fuel appliances)
- Fixed ventilation present and unblocked
- Flue condition and routing
- Stove installation and condition
- Gas appliance condition and installation
- Exhaust system integrity
But remember — the BSS is every four years. Things can deteriorate between examinations. Regular self-checks throughout the year are your responsibility.
Recommended Kit
- CO alarm (BS EN 50291): Minimum one near your solid fuel stove. Ideally one in each living/sleeping area. Budget around £15-30 each.
- Spare batteries: If your alarm is battery-powered, keep spares aboard.
- Flue thermometer: A magnetic thermometer on the flue pipe helps you burn efficiently and spot problems (a cold flue means poor draught).
- Stove rope and cement: For maintaining door seals. Keep some aboard for quick repairs.
Find Safety Services
Need a BSS examiner, gas engineer, or stove installer near your mooring? Search our directory for narrowboat services across the UK canal network.