Ten-year replacement set, five units

Alarms have a service life, usually ten years from manufacture rather than from installation, and a house fitted out in one go reaches the end of it in one go. Replacing them one at a time as each starts its end-of-life chirp takes eighteen months and half a dozen trips.

Five sealed combination smoke and CO units, wireless interlinked and pre-paired, which covers a four-bedroom house on two floors with a basement. Date-stamped on the body so the next replacement is a glance rather than a guess.

Fitting and care. Take the old units down first and check the date on each one — you will often find they were not all fitted in the same year, which is the thing this set exists to stop happening again. Pair the new ones on a table before any go up. Write the install date on every body with a marker. Set a calendar reminder for nine years and six months, not ten, so the replacement is planned rather than triggered by chirping. Dispose of the old units at an electrical recycling point.