Carbon Monoxide Safety: Alarm Placement, Flue Checks & Landlord Obligations

Quick Answer: Carbon monoxide (CO) is an odourless, colourless gas produced by incomplete combustion. CO alarms must be fitted in any room containing a fixed combustion appliance (gas boiler, gas fire, wood burner, oil boiler) under the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (England). Alarms must comply with BS EN 50291-1. Scotland has required CO alarms since 2022 under the Tolerable Standard. Landlords must install CO alarms and test them at the start of each tenancy.

Summary

Carbon monoxide poisoning kills around 40–50 people in the UK each year and causes hundreds more to be hospitalised or suffer lasting health effects. The tragedy is that most of these deaths are preventable with a functioning CO alarm. Despite this, CO alarm fitting is inconsistently applied, and many gas engineers still don't advise clients strongly enough about the need for CO detection adjacent to every combustion appliance.

CO is produced when any fuel (gas, oil, wood, coal) burns incompletely due to restricted air supply, incorrect burner adjustment, or a blocked or damaged flue. The critical danger is that CO has no smell, taste, or colour — by the time symptoms appear (headache, dizziness, nausea), significant exposure has already occurred. People have died without waking from sleep because CO levels were not high enough to trigger obvious symptoms before incapacitation.

For gas engineers in particular, understanding CO — its sources, how to test for it, and how to advise clients — is part of professional competence. Checking combustion efficiency and CO levels in flue gases at every service and installation is best practice, and identifying sources of CO risk before they become dangerous is part of the duty of care to clients.

Key Facts

• CO sources — any combustion appliance: gas boiler, gas fire, gas cooker (hob and oven), wood burner, multi-fuel stove, oil boiler, solid fuel Aga/Rayburn, paraffin heater, portable BBQs/generators (never use indoors)
• CO exposure limits — HSE Workplace Exposure Limit: 20 ppm (TWA over 8 hours), 100 ppm STEL (15-minute short-term exposure limit); symptoms begin at sustained levels of 50–100 ppm; life-threatening above 400 ppm
• CO alarm triggering levels — BS EN 50291-1 alarms are designed to alarm between 50–150 ppm depending on exposure duration; they do not alarm at the very low levels the HSE considers hazardous for long exposure (10–20 ppm)
• BS EN 50291-1:2010 — the standard for CO alarms for domestic use; current edition requires alarms to be tested to specific performance criteria
• BS EN 50292:2013 — siting and installation guide for CO alarms; covers placement principles
• Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (England) — landlords must fit a CO alarm in every room with a fixed combustion appliance (not just boilers — includes gas fires, wood burners, open fires)
• Scotland — Tolerable Standard 2022: CO alarm required in any room with a fuel-burning appliance; alarms must be interlinked with smoke alarms
• Wales — similar requirements under The Renting Homes (Fitness for Human Habitation) Regulations
• Testing at start of tenancy — landlords must test CO alarms at the start of each tenancy to confirm they work
• Alarm lifespan — CO alarms have an electrochemical sensor that degrades over time; typically rated for 5–7 years; always check manufacture date and replace at end of rated life
• Combination smoke/CO alarms — acceptable and increasingly popular; must meet BS EN 50291-1 for CO detection AND BS EN 14604 for smoke detection; both standards apply independently
• Flue CO testing — at every boiler service, measure CO and CO₂ in flue gases with a calibrated combustion analyser; a CO/CO₂ ratio >0.004 (4:1000 or 0.4%) indicates poor combustion that needs investigation
• Room CO testing — a CO level above 10 ppm in the room during appliance operation indicates a leak or flue spillage — investigate immediately
• RIDDOR — CO-related illnesses (from a work-related cause) are RIDDOR reportable; suspect CO poisoning from a gas appliance should be reported to the HSE

Quick Reference Table

Detailed Guidance

CO Alarm Placement

BS EN 50292 recommends:

• One CO alarm per floor in a property
• In any room with a fixed combustion appliance
• Ceiling or wall mounted, maximum 150mm from ceiling for wall-mounted
• Not directly above or immediately adjacent to the appliance (where CO/CO₂ concentrations may be high during normal operation, causing nuisance alarms)
• Not in bathrooms or rooms with high humidity
• Not directly above a cooker or toaster

Minimum required positions (England, Smoke and CO Alarm Regulations 2022):

• In every room with a fixed combustion appliance (gas boiler room, room with gas fire, room with wood burner)
• This is the legal minimum for landlords; best practice is one per floor regardless

Common placement for boilers: if the boiler is in a kitchen, position the CO alarm on the ceiling between the boiler and the sleeping areas — typically in the hallway outside the kitchen, or on the landing if the boiler is on the ground floor. This provides early warning before CO reaches sleeping occupants.

For wood burners and multi-fuel stoves: fit a CO alarm in the same room as the appliance AND on the landing above. CO from stoves can accumulate in sleeping areas via air leakage around doors.

Flue Spillage Testing

Gas appliances with open flues (traditional gas fires, back boilers, older boilers) can "spill" combustion products into the room if the flue is blocked, the draught is insufficient, or the appliance is in a very airtight room.

Open flue check procedure (at every service):

1. With appliance running, hold a smoke match or smoke pencil at the draught diverter or draught hood — smoke should be drawn in
2. If smoke is blown out, there is flue spillage — investigate: blocked flue, flue terminal obstruction, inadequate air supply
3. Close all doors and windows — if spillage stops, the room is too airtight; ventilation is required
4. If spillage persists, use a CO analyser at the draught diverter — any reading above 10 ppm suggests a spillage issue; above 50 ppm is potentially dangerous

Room-sealed appliances (balanced flue boilers) cannot spill combustion products into the room — all combustion air is drawn from outside and all flue gases are discharged outside. However, a cracked or damaged heat exchanger can still allow CO into the heated water (not typically dangerous) or, more rarely, into the room if the flue system has a defect.

Combustion Analysis at Service

Every service of a gas appliance should include combustion analysis with a calibrated flue gas analyser. Check:

• CO in flue gases: natural gas appliances should produce very low CO in the flue (typically <100 ppm); higher levels indicate incomplete combustion
• CO₂: typically 8–10% for natural gas; outside this range indicates air/gas mixture is incorrect
• CO/CO₂ ratio: the ratio of CO to CO₂ in the flue; the ratio should be <0.004 (i.e., CO is less than 0.4% of CO₂ reading); higher ratios indicate poor combustion that could lead to dangerous CO levels if the appliance degrades further
• Flue gas temperature: high temperature (>120°C above ambient on a condensing boiler) may indicate a scaling or heat exchanger issue

A CO/CO₂ ratio above 0.004 is a warning sign. Above 0.01 (1%), investigate immediately. This is an "at risk" (AR) situation under the Gas Industry Unsafe Situations Procedure.

Suspected CO Poisoning — What to Do

If you suspect a client has been exposed to CO (they report persistent headaches, flu-like symptoms without fever, symptoms that improve when away from home):

1. Advise them to leave the property immediately — get into fresh air
2. Do not re-enter until the source has been identified and isolated
3. Seek medical attention — CO poisoning requires hyperbaric oxygen treatment in severe cases; blood CO level (carboxyhaemoglobin) should be measured at A&E
4. Call 999 if symptoms are severe (confusion, loss of consciousness, vomiting)
5. Call the gas emergency line (0800 111 999 for National Gas Emergency) to report a suspected gas/CO leak
6. If you are the attending gas engineer: test ambient air CO levels with a CO detector, find and isolate the source, classify as ID or AR as appropriate, label the appliance, and record everything

Landlord Obligations (England)

Under the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022:

• CO alarms must be present in every room with a fixed combustion appliance (this includes: gas boiler in kitchen, gas fire in living room, wood burner in any room)
• Alarms must be working at the start of each new tenancy (landlord responsibility to test before tenancy begins)
• Tenants are responsible for testing during the tenancy and reporting failures to the landlord
• Landlords must repair or replace faulty alarms promptly when informed
• Penalty: remediation notice from local council; if not complied with, civil penalty of up to £5,000

Best practice for landlords: document CO alarm testing in the tenancy check-in report with a photograph showing the alarm and its battery/test status. Replace CO alarms at their end-of-life date (check manufacture date on the back).

Frequently Asked Questions

Where exactly should I fit a CO alarm in a room with a boiler?

Ceiling-mounted between 300mm and 3m from the boiler (not directly above it — that position can give elevated readings during normal operation). For a boiler in a kitchen, fit on the ceiling in the hallway just outside the kitchen door, so it protects the sleeping areas above without being in the highest-activity area.

Can a CO alarm substitute for a gas leak detector?

No. CO alarms detect carbon monoxide — a combustion product. They do not detect natural gas or LPG leaks (methane, propane, butane). A gas alarm for unburned gas is a separate device (methane detector or combined unit). CO alarms and gas alarms are different products with different sensors.

My client's CO alarm keeps going off but there's no source I can find — why?

Several possibilities: (1) the alarm is at end of life and the sensor has degraded, causing false triggers — check the manufacture date and replace if >5–7 years old; (2) low-level CO from a vehicle in a garage below the room; (3) neighbours burning waste near air bricks; (4) a very minor flue defect not detectable on visual inspection — use a combustion analyser and do a full flue spillage test; (5) the alarm is responding to solvents or cleaning products (unlikely with a good-quality alarm, but possible with cheaper units).

Is a CO alarm on its own sufficient protection?

No — alarms are the last line of defence. The primary protection is: correct installation and commissioning of all gas appliances by Gas Safe registered engineers, annual servicing, working flue and ventilation, and correct combustion at the burner. A CO alarm is essential but should not substitute for maintaining appliances correctly.

Regulations & Standards

• Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (England) — landlord obligations; CO alarms in all rooms with fixed combustion appliances

• BS EN 50291-1:2010 — electrochemical CO alarms for domestic use; performance requirements

• BS EN 50292:2013 — siting and installation guide for CO alarms in domestic properties

• Gas Safety (Installation and Use) Regulations 1998 — gas appliance safety requirements

• Housing (Scotland) Act 2014 and Tolerable Standard Regulations 2022 — Scottish CO alarm requirements

• IGEM/G/SR1 — Gas Industry Unsafe Situations Procedure (CO-related classification)

• RIDDOR 2013 — reporting of CO poisoning incidents from work-related causes

• CO-Gas Safety — Consumer and Professional Guidance

• GOV.UK — Smoke and CO Alarm Regulations for Landlords

• Gas Safe Register — CO Safety

• NHS — Carbon Monoxide Poisoning

• gas safe requirements — Gas Safe registration for appliance work

• boiler installation — commissioning checks including CO analysis

• smoke alarms — combined smoke and CO alarm requirements