Smoke and CO alarm, three-pack

Three sealed combination smoke and carbon monoxide alarms, which is the minimum for a typical two-storey house with a basement: one per level, plus one outside each sleeping area if the layout needs it.

Sealed ten-year batteries, photoelectric smoke sensing, electrochemical CO cell, voice announcement of the hazard type and location. Buying the three together is about eight per cent under the single price, and it means every alarm in the house reaches end of life in the same month rather than one at a time for a decade.

Fitting and care. Fit all three on the same day and write that date on each body, because the value of buying a set is that they expire together. One per storey minimum, plus one outside each sleeping area; if a bedroom door is normally closed, the alarm outside it will not wake the person inside, so consider a unit in the room as well. Test all three monthly. Vacuum twice a year. Never paint them and never move one into a kitchen to solve a coverage gap.