Smoke and CO detectors: interconnected, 10-year sealed, and where they go

Replace every smoke detector older than 10 years (date on the back). Buy 10-year sealed lithium units; no more 3 a.m. chirps. Hardwired ones are interconnected (one goes off, all go off) and need a replacement with the same connector or a universal adapter; wireless-interconnect battery units exist for houses without the wiring.

Placement: one in every bedroom, one outside each sleeping area, one per floor including the basement, none within 10 ft of a cooking appliance (use a heat alarm in the kitchen). CO detectors on every floor and near bedrooms, at knee height or plug-in level is fine; CO mixes evenly with air.