Carbon Monoxide Signs Every Property Owner Should Know

A boiler can appear to be working normally while producing a serious risk. Carbon monoxide has no smell, taste or colour, so tenants and homeowners cannot rely on their senses to detect it. The warning usually comes from a working alarm, an appliance fault found during servicing, or symptoms that should never be ignored.

For landlords, this is a legal compliance issue as well as a duty of care. For homeowners, it is a straightforward reason to keep gas appliances serviced and act quickly when something changes. A valid gas safety check, suitable alarms and prompt repairs are practical safeguards that protect people and property.

What is carbon monoxide?

Carbon monoxide, often called CO, is a poisonous gas made when fuel does not burn fully. Gas, oil, solid fuel and wood-burning appliances can all create CO if they are faulty, badly maintained, incorrectly fitted or poorly ventilated.

In a home, possible sources include gas boilers, fires, cookers, water heaters and blocked flues or chimneys. The appliance itself is not always the only issue. Damaged seals, a lack of ventilation, poor installation and flue problems can all affect how combustion gases leave the building.

A modern gas boiler in good condition should operate safely. However, no appliance should be assumed safe simply because it still heats water or radiators. Annual checks and servicing help identify faults before they develop into a dangerous situation.

Carbon monoxide symptoms and warning signs

CO poisoning can affect people differently. Common symptoms include headaches, dizziness, nausea, breathlessness, tiredness, confusion and chest or stomach pain. They may feel similar to flu symptoms, but without a high temperature.

A useful clue is when symptoms improve after leaving the property, or when more than one person in the household feels unwell at the same time. Babies, children, older people and anyone with heart or breathing conditions may be affected more severely. Pets can also become ill.

Physical signs around an appliance can suggest a problem, although their absence does not prove everything is safe. Look out for yellow or lazy flames instead of crisp blue flames on gas appliances, staining or soot around an appliance, frequent pilot-light problems, excessive condensation, or unfamiliar fumes. Do not investigate by touching, dismantling or trying to repair the appliance yourself.

A carbon monoxide alarm is the most reliable early warning device for occupants. It is not a substitute for appliance maintenance, but it gives people time to leave and seek help if CO is detected.

What to do if you suspect carbon monoxide

Treat a sounding CO alarm, or symptoms that may be linked to CO, as urgent. Do not wait to see whether the alarm stops or symptoms pass.

If it is safe to do so, follow these steps:

• Turn off gas appliances if you can do this without putting yourself at risk.

• Open doors and windows on your way out to help ventilate the property.

• Leave the building immediately and keep everyone, including pets, outside.

• Call the National Gas Emergency Service on 0800 111 999 from outside. If anyone is seriously unwell, call 999 for urgent medical help.

Do not re-enter the property or use the suspected appliance until you have been told it is safe. A qualified Gas Safe registered engineer should inspect the appliance, associated pipework and flue to find the cause. Replacing an alarm battery or resetting the unit does not resolve the source of a CO leak.

Carbon monoxide alarms in rented homes

Landlords in England must provide a carbon monoxide alarm in every room used as living accommodation that contains a fixed combustion appliance, excluding gas cookers. This includes rooms with a gas boiler, gas fire, wood burner or other relevant fixed appliance.

The alarm must be in working order at the start of each new tenancy. Tenants should test alarms regularly and report any fault promptly. Once a landlord has been told that an alarm is not working, they must take reasonable steps to repair or replace it.

Placement matters. Follow the alarm manufacturer's instructions, as the recommended position varies by model and appliance type. Avoid treating an alarm as a box-ticking exercise. An alarm that is badly positioned, out of date or has a flat battery may not provide the protection occupants expect.

Landlords should keep a simple record of alarm checks, reported faults and replacements. It is useful evidence of responsible management, particularly where several properties or managing agents are involved.

Gas safety checks are not optional for landlords

A carbon monoxide alarm does not replace a landlord gas safety check. In rented properties with gas appliances, landlords must arrange a safety check every 12 months by a Gas Safe registered engineer. The check covers relevant gas appliances, pipework, flues and safety devices.

After the inspection, the landlord receives a Landlord Gas Safety Record, often referred to as a CP12 certificate. Existing tenants must receive a copy within 28 days of the check. New tenants must receive it before they move in, and landlords must retain records for at least two years.

The annual inspection is designed to assess safety, not to guarantee that an appliance will never break down. That distinction matters. A boiler may pass a safety check and later develop a fault, particularly if it is ageing or heavily used. Servicing at the manufacturer-recommended interval, alongside the annual safety check, is the sensible approach for reliability as well as compliance.

Where an engineer identifies an unsafe appliance, it may need to be turned off and labelled to prevent use. This can be inconvenient during a tenancy, but allowing an unsafe appliance to remain in service is not an acceptable alternative. Arrange repair or replacement quickly and keep tenants clearly informed.

Practical prevention for homeowners and tenants

Most CO risks can be reduced with routine attention rather than complicated changes. Book regular boiler servicing, use a suitably certified carbon monoxide alarm, and ensure ventilation openings are not blocked by furniture, storage or decorating work.

Never use a gas cooker, barbecue, patio heater or generator to heat an indoor room. Portable fuel-burning equipment belongs outdoors or in the setting specified by its manufacturer. Even a short period of indoor use can create dangerous levels of CO.

Do not cover, block or alter boiler flues, air bricks or ventilation grilles. If building work, a new extension, external cladding or a replacement kitchen changes the area around an appliance or flue, have the installation checked. What looks like a cosmetic change can affect ventilation or safe discharge of combustion gases.

Tenants should report warning signs early. A boiler losing pressure repeatedly, a fire that will not stay lit, unusual staining, damaged flue terminals or an alarm fault should not be left until the next routine visit. Early reporting gives the landlord time to arrange a proper inspection before the issue becomes more disruptive.

When a boiler service and a gas safety check differ

The terms are often used as if they mean the same thing, but they have different purposes. A landlord gas safety check focuses on whether the relevant gas installation is safe at the time of inspection. A boiler service is a more detailed maintenance visit that follows the appliance manufacturer's requirements and can support efficiency, performance and lifespan.

For a rental property, the right arrangement depends on the appliance, its service history and the manufacturer's instructions. Combining the annual gas safety check with a boiler service is often efficient, especially at a tenancy changeover. However, a recently serviced boiler may not need duplicate work simply because the legal safety check is due. A Gas Safe registered engineer can advise on the appropriate work and provide the required documentation.

Keep safety work prompt and documented

For landlords managing a changeover, timing is critical. A certificate issued after a tenant has moved in does not meet the requirement to provide the record before occupation. Booking ahead is the easiest option, but urgent appointments are useful when a tenancy date changes or a fault is reported without warning.

Bournemouth Gas Certificates provides gas safety checks and related domestic gas work across Bournemouth, Poole and Christchurch, with same-day certificate issuing after a satisfactory inspection. The priority should always be the condition of the installation, not simply obtaining paperwork quickly.

If an alarm sounds, a tenant reports symptoms, or an appliance starts behaving differently, act on it that day. Carbon monoxide safety depends on early action, competent inspection and never taking a silent risk for granted.