Smoke and carbon monoxide alarms are the cheapest life-safety equipment in the house and the most casually neglected: unplugged after a burnt dinner, expired a decade past their date, or absent from the bedrooms where fatal fires start at night. Smoke detector placement, sensor type, interconnection, and the ten-year clock are the whole discipline, and it fits in one reference. CO placement rides along, with its combustion context covered in the carbon monoxide safety basics.
Placement: the geography of survival
The smoke-alarm map follows sleep, because the fatal fires are the ones that start while the house is unconscious: an alarm inside every bedroom, one outside each sleeping area (the hallway serving the bedrooms), and one on every level, basement included, with the basement unit at the stairway’s ceiling. Mounting follows smoke’s physics upward: on the ceiling, or high on the wall within a foot of it, away from the dead-air corners where ceiling meets wall, and clear of supply registers and ceiling fans whose airflow washes smoke past the sensor. The misfire geography matters equally, since nuisance alarms are why units get unplugged: distance from the stove (ten feet is the working rule, with photoelectric or hush-button units for kitchen-adjacent spots), out of bathroom steam plumes, and out of garages and unconditioned attics where temperature and exhaust exceed their ratings, heat alarms serving those spaces instead. CO alarms overlay a similar map, every level and outside sleeping areas, per their instructions on height, since CO mixes with air rather than rising like smoke.
Types: two sensors, two fires
The two smoke technologies specialize. Ionization sensors respond fastest to fast, flaming fires, the wastebasket and the grease flare. Photoelectric sensors respond fastest to smoldering fires, the cigarette in the cushion, the overheated cord behind the couch, the failing connection’s slow beginning, which produce hours of deadly smoke before flame. Each meaningfully lags on the other’s specialty, which is why the standing recommendation is both in every home: dual-sensor units, or a deliberate mix through the house, with photoelectric favored near kitchens for its nuisance resistance. Combination smoke/CO units are legitimate where listed, provided the household knows its two distinct alarm patterns, and the specialty roster rounds out with heat alarms for garages and strobe/bed-shaker units where hearing needs them.
Interconnection and power
Interconnection, all alarms sounding when any triggers, is the difference between a basement alarm and a basement alarm that wakes the upstairs bedrooms, and modern codes require it in new work. Hardwired-with-battery-backup is the construction standard; wireless-interconnect battery units retrofit the same behavior into existing houses without pulling cable, one of the highest-value safety upgrades an older home can buy. Power discipline is the famous weak link: batteries changed on the clock-change ritual (or bought as ten-year sealed units that retire with the alarm), and the absolute rule that a chirping or nuisance-prone alarm gets fixed or relocated, never shelved, the disabled-alarm statistic being a grim constant in fire investigations.
The ten-year clock and the calendar
Sensors expire: smoke alarms at ten years from the manufacture date printed on the back, CO alarms at five to ten by model, regardless of batteries and regardless of the test button’s cheerful beep, which tests the horn and circuit rather than the sensor’s remaining sensitivity. The ownership calendar is short: monthly, press test on a rotating sample; twice yearly, batteries with the clock change, plus a vacuum of the vents, since dust and cobwebs cause both nuisance and silence; yearly, read a date or two; at ten years or any unknown date, replace, and a house purchase resets every clock to “unknown.” Renters inherit the same map with the landlord’s hardware, and the panel-labeling afternoon pairs naturally with a whole-house alarm census, dates written on each unit in marker where the next tester can see them.
The bottom line
Every bedroom, every hallway, every level; both sensor technologies; interconnected so the basement can wake the bedroom; batteries on the clock change and the whole unit at ten years. The equipment costs less than a tank of gas per decade, and its entire failure mode is being ignored, which is the one defect this reference cannot repair.