Correctly positioning gas and carbon monoxide detectors in your home is not a matter of aesthetic preference, but a decision governed by the laws of physics and fluid dynamics. Placing a sensor in the wrong location can delay alarm activation by crucial minutes, rendering even the most advanced device ineffective when a leak occurs.
The Physics of Gas Density: Why Height is Crucial
To understand where to install a detector, you must understand the behaviour of the target gas relative to ambient air. Air has an average molar mass of approximately 29 grams per mole (g/mol). Any gas with a molar mass significantly lower than this will rise, while heavier gases will sink and pool at floor level.
- Natural Gas (Methane): With a molar mass of about 16 g/mol, methane is much lighter than air. When a leak occurs, it rises rapidly toward the ceiling, filling the room from the top down.
- Liquefied Petroleum Gas (LPG/Propane-Butane): Propane (44 g/mol) and butane (58 g/mol) are much heavier than air. These gases sink immediately, pooling along the floor, in basement wells, and in low-lying cavities.
- Carbon Monoxide (CO): Carbon monoxide has a molar mass of 28 g/mol, making it marginally lighter than air. However, because it is usually produced by hot combustion appliances, it rises initially with the warm thermal currents (convection) before diffusing evenly throughout the living space.
Where to Position Carbon Monoxide (CO) Detectors
Carbon monoxide is an invisible, odourless, and highly toxic gas. Because it mixes relatively evenly with air at room temperature, the goal is to measure the air in your breathing zone. For maximum protection, install detectors on every level of the home, particularly near sleeping quarters.
In bedrooms, the detector should be placed at sleeping height—roughly level with the mattress. In living areas, eye level (approximately 1.5 metres from the floor) is optimal. If you are installing a detector in a room with a fuel-burning appliance (such as a boiler, fireplace, or gas stove), place the device between 1 and 3 horizontal metres away from the potential source. Placing it closer than 1 metre can lead to false alarms caused by tiny, non-hazardous traces of combustion products released during start-up.
Positioning Natural Gas and LPG Sensors
Due to the stark density differences of explosive gases, a single height recommendation does not exist. You must select the mounting height based on the specific fuel source used in your home.
For methane (piped natural gas), the sensor must be mounted high on the wall, approximately 30 centimetres below the ceiling. Do not mount it directly in the apex of a pitched roof, as "dead air" pockets can prevent the gas from reaching the sensor.
For LPG (bottled gas), the detector must be positioned low down, about 30 centimetres above the floor. Avoid placing these sensors near floor drains, ventilation grilles, or external doors where drafts could dilute the accumulating gas before the sensor can detect it.
Avoiding Dead Air Zones and Interference
Air circulation patterns significantly affect how quickly a sensor detects danger. To ensure reliable operation, you must avoid installing detectors in areas where air stagnates or where environmental factors can damage the sensor chemistry.
- The 30-Centimetre Rule: Avoid placing ceiling-mounted detectors within 30 centimetres of any wall, and wall-mounted detectors within 30 centimetres of the ceiling. These corners are "dead air" zones where air currents rarely circulate.
- Drafts and Ventilation: Do not place detectors directly next to windows, extractor fans, exterior doors, or heating vents. The constant flow of fresh or moving air will dilute the target gas around the sensor, delaying or preventing an alarm.
- Humidity and Cooking Byproducts: Keep detectors at least 3 metres away from bathrooms and cooking appliances. High humidity, steam, and airborne cooking oils can coat the electrochemical or catalytic sensors, desensitising them over time or causing frequent false alarms.
A Quick Reference Placement Checklist
To ensure your home protection system is fully optimised, verify your installation against this quick checklist:
- Methane detectors: High up (30 cm below ceiling), close to the gas source but away from direct drafts.
- LPG detectors: Low down (30 cm above floor), avoiding low areas prone to dampness.
- Carbon monoxide detectors: At breathing height in bedrooms and living rooms; 1 to 3 metres away from boilers.
- Maintenance: Vacuum the sensor grilles gently every six months to remove dust barrier layers, and test the internal circuitry monthly using the test button.
