When buying battery-powered CO Detectors and Smoke Detectors it is highly recommended that you purchase "sealed unit" long-life battery alarms.
These alarms do not require replacement batteries each year and when the built-in battery dies, it is the end of the unit's life. The problem with replaceable battery powered units is that we often find detectors that are far past their "use by" date (usually 10 years) because the tenants and landlords have just been changing the battery each year without checking that the detector is still in date. These long-life alarms also avoid landlords having to rely on tenants to change the batteries when they die (which some tenants just don't do). Another problem these detectors help to avoid is where a tenant disconnects the battery to stop the alarm going off.
In short, it makes a lot of sense to purchase these types of alarms over the old replaceable battery alarms - especially considering that the cost difference is only £5 or £10.
We recommend the following alarms: