Carbon Monoxide Detector Placement and Maintenance

A carbon monoxide detector is the only reliable warning system for CO — the gas itself provides no sensory cue. Understanding where to place detectors, how many to install, and how to maintain them correctly is as important as having them at all. A CO detector in the wrong location, or one with a depleted battery or expired sensor, offers false security.

Where CO Comes From — and Why Placement Matters

CO is produced by incomplete combustion from:

• Gas boilers and central heating systems
• Gas cookers and hobs
• Gas fires and flueless heaters
• Oil boilers
• Wood-burning and multi-fuel stoves
• Open fires and fireplaces
• Generators, petrol engines indoors
• Charcoal barbecues used indoors

CO rises and disperses throughout a room relatively quickly, but concentrations are highest closest to the source. Detectors should be positioned to catch accumulating CO before it reaches dangerous levels in sleeping and living areas.

How Many Detectors Do You Need?

Property Type	Minimum Recommendation
Single-storey flat/apartment	1 detector
Two-storey house with gas appliances on ground floor	1 per floor (minimum 2)
House with gas boiler and gas fire on separate floors	At least 1 near each combustion appliance
House with basement boiler	1 in basement, 1 on each additional floor
Any property with sleeping area separate from appliances	At least 1 outside each sleeping area

General rule: One detector per floor, and one within 3 metres of any significant combustion appliance.

Room-by-Room Placement Guide

Hallway / Landing

The most important location. Detectors in hallways serve multiple rooms:

• Ground floor hallway — catches CO from kitchen/utility room, garage, boiler cupboard
• Upstairs landing — protects sleeping rooms; CO entering from downstairs rooms will reach here
• Position: On the wall or ceiling, away from draughts (exterior doors, stair wells that create air movement)

Bedroom

If you sleep with the door closed, a hallway detector may not protect you:

• Install a detector inside or immediately outside any bedroom regularly occupied
• Particularly important in properties with gas fires, wall heaters, or gas boilers in adjacent utility rooms

Boiler Room / Utility Room

The room housing the boiler is the highest-risk single location:

• Install a detector within 3 metres of the boiler
• If the boiler is in a cupboard, install a detector on the wall immediately outside it — CO sensors need adequate airflow to function

Living Room

If you have a gas fire, open fireplace, or wood burner:

• Install a detector within the room, away from the appliance itself (not immediately above the appliance — heat can interfere with electrochemical sensors)
• Ideal position: 1–3 metres from the appliance at ceiling height or shelf height

Kitchen

The kitchen can be a source of CO from cookers and hobs, but CO detectors should not be installed directly above a cooker:

• Cooking combustion products can cause false alarms with electrochemical sensors
• Position at least 1 metre from the cooker, away from direct cooking steam
• A kitchen detector is secondary to a hallway detector for most homes

⚠️ Do not place a CO detector directly next to or above a gas appliance, in a particularly steamy area, or in a cupboard. These positions can cause false alarms or reduce detection accuracy. The detector's purpose is to catch CO accumulating in the room air, not to monitor the appliance directly.

Height — Where on the Wall or Ceiling?

Unlike smoke detectors, carbon monoxide sensors work at any height because CO mixes with room air at relatively uniform concentrations throughout a room. Current UK guidance (British Standard EN 50291) and NFPA (US) guidance recommends:

Position	Notes
Ceiling or high wall (within 30cm of ceiling)	Good general position for most rooms
Breathing height (head height when sleeping)	Recommended for bedrooms specifically
Not at floor level	Avoid — CO does not pool at the floor; some interference from floor drafts
Not directly above heat sources	Heat can affect sensor accuracy

Detector Types

Type	How It Works	Pros/Cons
Electrochemical sensor	Chemical reaction with CO generates electrical current	Most accurate; used in professional-grade units; sensor has a finite life
Biomimetic sensor	Gel darkens in presence of CO	Lower cost; does not provide digital readout
Metal oxide semiconductor	Resistance changes with CO	Less common in home units

Most home CO detectors use electrochemical sensors. The sensor itself has a service life of approximately 5–7 years regardless of battery condition — a detector that works (sounds when tested) but has an expired sensor will not reliably detect CO. Check the manufacture date on the back of the unit.

Maintenance Schedule

Task	Frequency
Test alarm button	Monthly
Replace battery (battery-powered units)	When low-battery alert sounds, or annually
Clean exterior with dry cloth	Annually
Check manufacture date — replace if 5–7 years old	At manufacture date check
Replace unit entirely	Every 5–7 years (sensor life)

Testing the alarm: Press and hold the test button for 5–10 seconds. The alarm should sound. This tests the alarm circuit and battery, but does not test whether the sensor is detecting CO accurately. Sensor accuracy degrades over time regardless of the test result.

Never spray cleaning products, air freshener, or aerosols near a CO detector — these can damage the electrochemical sensor permanently.

When the Alarm Sounds

A triggered CO alarm requires immediate action — do not assume it is a false alarm:

1. Do not turn off the alarm — you need the alert.
2. Move everyone outside — including pets.
3. Leave doors open as you exit to ventilate.
4. Call 999 (UK) or 911 (US) from outside.
5. Call the gas emergency number — UK: 0800 111 999.
6. Do not re-enter until emergency services confirm it is safe.

If anyone has symptoms (headache, nausea, dizziness), seek medical attention even if they feel better in fresh air.

Combined CO and Smoke Detectors

Combination units that detect both smoke and CO are available. These are acceptable for CO detection provided they meet BS EN 50291 (CO) and BS 5446-1 (smoke) standards. Be aware that the CO sensor still has a limited service life — check the manufacture date.

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Quick Reference

Location	Priority	Notes
Upstairs landing	High	Protects sleeping areas
Ground floor hallway	High	Near boiler, kitchen, garage
Within 3m of boiler	High	Highest-risk single appliance
Each bedroom (door closed)	Medium	Particularly if gas appliances nearby
Living room with gas fire	Medium	Away from the appliance itself
Replace every 5–7 years	Mandatory	Sensor life expires regardless of battery
CO alarm sounds	Evacuate immediately	Do not investigate first