How Far Should Carbon Monoxide Detector Be From Boiler. 553885 Carbon Monoxide Alarm Required lists each place where a CO detector must be mounted. In close proximity to any fuel-burning appliance.

The alarm should be marked with British Standard EN 50291 and a British or European approval mark such as the Kitemark. The main thing that you are trying to avoid here is false alarms. Combination Smoke CO Alarm.
So it should be outside the closet.
The alarm should be marked with British Standard EN 50291 and a British or European approval mark such as the Kitemark. We recommend the use of an audible carbon monoxide detector rather than one that just changes colour because it will alert you straightaway and wake you if you are asleep. In excessively humid areas such as your bathroom. Combination Smoke CO Alarm.
