Carbon monoxide alarm, three-pack

Carbon monoxide is heavier work to cover than smoke, because the source is usually an appliance and the people at risk are usually asleep on another floor. Three units covers the standard advice: one outside each sleeping area, one on the level with the fuel-burning appliance.

All three with digital displays and peak-level memory, ten-year sensor life, battery backup. Same end-of-life date across the set, which is the practical reason to buy them together.

Fitting and care. Breathing height in a hallway is the right position; fifteen feet from any fuel-burning appliance avoids nuisance readings at start-up. Clear the peak memory on all three after you fit them so you have a clean baseline, then read them before you clear again. Fit on one day, record the date, replace as a set. Test monthly. If any unit shows a persistent reading above 30 ppm, that is a call to the gas supplier with a number in hand, not something to watch.