Common mistake 9 of 9 · Safety
Trusting a CO alarm that can’t do its job
The mistake: Carbon monoxide has no smell and no colour — the alarm is your only sense for it, and placement, position and expiry quietly decide whether it works.
Carbon monoxide gives you nothing to work with: no smell, no colour, no irritation — which is why households buy a CO alarm and then file the whole subject away as handled. Two quiet mistakes undo that purchase more than any other: putting the alarm where it can’t read the gas, and keeping it past the life of its sensor.
Why it backfires
Placement is the first failure. An alarm squeezed right next to the boiler nuisance-trips — and the most dangerous outcome of a false alarm isn’t the noise, it’s the person who pulls the battery. Across the room but behind a curtain, in a shelf nook, or on the wall the door opens against, the alarm sits in dead air and reads the room late or not at all. Expiry is the second: the sensor inside a CO alarm is a consumable, with a service life manufacturers stamp on the back (commonly in the five-to-ten-year band). A test button proves the battery and the sounder — it cannot prove the sensor still detects anything. Combination smoke-and-CO units retire at the shorter of the two lives, which is almost always the CO side.
The fix
Read the alarm’s own instructions once and follow the distance band they give — UK landlord guidance commonly puts one within one to three metres of the appliance, and the same logic serves a homeowner. CO mixes evenly with the air, so ceiling or high wall is fine (it doesn’t stratify like smoke), but never behind furniture, never in a corner the air doesn’t move through, and not directly beside an extraction vent or window that would dilute a reading. If the alarm false-alarms next to the boiler, don’t disable it — move it to the far end of the same room per the manual, and book the boiler a service, because repeated nuisance alarms from a flue are a symptom, not a glitch. Then check the replace-by date printed on the unit; if it has passed, the whole alarm is replaced — on most domestic models the sensor can’t be swapped.
Keep it from happening
Buy alarms with a sealed long-life battery and an end-of-life chirp, put the expiry date in your phone calendar the day you unpack it, and fold the test into the monthly alarm habit so a dying alarm is heard, not discovered. Have fuel-burning appliances serviced annually by a registered engineer — Gas Safe-registered in the UK — because an alarm is the last line of defence, never the maintenance. Where the law already sets the minimums, the UK alarm-law guide has them, and the expiry guide explains how to read the dates nobody does.
Sources
Next
Related on this desk.
General information, not professional advice. Homes differ — if a job is beyond your confidence or the guide’s boundary, that is what tradespeople are for.