In England and Wales, a CO alarm is only mandatory if a solid fuel burning appliance, e.g. wood or coal, is installed.
In Scotland and Northern Ireland from October 2013, a CO alarm became mandatory when any type of fuel burning appliance is installed or replaced in a new or existing home. A CO alarm is also mandatory if a “flue in void” situation is present, where the flue cannot be adequately inspected.
EN50291 defines the CO level at which the device must and must not go into alarm. The higher the CO concentration, the quicker the device will give an alarm. This is based on the fact that the poisoning impact of CO is determined by CO concentration and exposure time.