The Edit — Home Safety
Smoke and CO Alarms: Sensor Type, Placement, Replacement
Photoelectric and ionisation sensors detect different fires, carbon monoxide needs its own sensor entirely, and every alarm on the market expires 10 years after manufacture whether it has ever sounded or not.
This guide is research-led: we have not handled these units ourselves. If you buy through our links we may earn a commission — it never changes a pick. How we choose
Every alarm on this page carries a manufacture date stamped on the back, and every one of them expires 10 years after it. The sensor degrades whether the alarm has ever sounded or not.
SC9120B Hardwired Smoke and Carbon Monoxide Alarm
Good fit You are replacing one unit in an interconnected chain
Not for There is no existing wiring to connect to
Kidde Smoke and Carbon Monoxide Detector, 30CUDR
Good fit No wiring, and you want both sensors in one unit
Not for A false alarm at 3am would end your patience
Siterlink Hardwired Interconnected Photoelectric Smoke Alarm
Good fit You want the sensor type printed rather than inferred
Not for You want a combination smoke and CO unit
First Alert Battery Combination Smoke and CO Alarm
Good fit A bedroom with no wiring and a fuel appliance nearby
Not for You want a hardwired interconnected system
First Alert Hardwire Combination Smoke and CO Alarm
Good fit You want the deepest evidence base available
Not for You have had nuisance alarms before
Kidde Smoke Detector, AA Battery-Powered, 20SDR
Good fit A room with no fuel-burning appliance anywhere near
Not for The room has a boiler, fire or attached garage
Two smoke sensors, and they are not interchangeable
Photoelectric sensing bounces light off smoke particles. It responds fastest to smouldering fires, which is what upholstery, bedding and electrical insulation do for a long time before they flame.
Ionisation sensing detects the change in a small ionised current. It responds fastest to fast-flaming fires with small particles, such as a pan of oil or paper.
Most domestic fatalities involve smouldering fires that fill a house with smoke overnight, and photoelectric alarms are the ones that respond to those first. They are also markedly less prone to nuisance alarms from cooking.
Dual-sensor alarms carry both. Where a listing names neither, you cannot tell what you are buying, and only 1 of the 6 products on this page states its sensor type at all.
| Sensor | Fastest on | Nuisance alarms |
|---|---|---|
| Photoelectric | Smouldering fires | Fewer |
| Ionisation | Fast-flaming fires | More, near kitchens |
| Dual sensor | Both | Depends on implementation |
Siterlink Hardwired Interconnected Photoelectric Smoke Alarm
One of the few listings that names its sensor type, and owners buy it specifically as a 10-year replacement.
Current price shown on Amazon. We may earn a commission.
| Sensor | Photoelectric, stated |
|---|---|
| Power | Hardwired, interconnected |
| Watch | Nuisance alarms reported by some |
This is the one listing here that names its sensor type, and it is photoelectric, which is the type worth choosing for a home.
Owners buy it specifically as a 10-year replacement, which is the maintenance job the section below is about. Nuisance alarms are reported by some, which is the failure mode that matters most and is covered further down.
Buy it if
- You want the sensor type printed rather than inferred
- You are replacing a whole expired set
Don't buy it if
- You want a combination smoke and CO unit
Carbon monoxide needs its own sensor
No smoke sensor detects carbon monoxide. It is a colourless, odourless gas produced by incomplete combustion, and a smoke alarm will sit silently through a lethal concentration of it.
That means a combination unit is genuinely two devices in one housing, and it means a smoke-only alarm in a room with a boiler, a gas fire, a wood burner or an attached garage is the wrong device.
CO alarms also carry their own expiry, commonly 7 years against the smoke sensor 10. On a combination unit, the shorter of the two ends the device. Four of the 6 alarms here detect both.
First Alert Battery Combination Smoke and CO Alarm
1,124 ratings on the battery combination unit, which is the one to fit where there is no wiring and both hazards apply.
Current price shown on Amazon. We may earn a commission.
| Power | Battery |
|---|---|
| Detects | Smoke and carbon monoxide |
| Suits | Bedrooms and hallways |
1,124 ratings on a battery combination unit, which is the sensible choice for a bedroom with no wiring in a house that burns any fuel.
Battery units are what make full coverage affordable. The trade is that you own the maintenance, and the test button is the only thing that tells you the sensor still works.
Buy it if
- A bedroom with no wiring and a fuel appliance nearby
- You want a familiar brand with real evidence
Don't buy it if
- You want a hardwired interconnected system
The 10-year rule is the one most people miss
Smoke sensors drift as they age. Manufacturers print a service life, universally 10 years from manufacture, and the date is on the back of the unit rather than on the box.
That matters when buying, because an alarm that sat in a warehouse for 2 years gives you 8. Check the stamp before fitting and write the replacement year on the case in marker.
Batteries are the shorter cycle. A sealed 10-year lithium unit removes that job entirely; a replaceable-battery unit needs an annual change and will chirp at 3am to tell you so.
Test every alarm monthly with the button. That tests the sounder and the circuit, not the sensor, which is precisely why the 10-year replacement is not optional.
First Alert Hardwire Combination Smoke and CO Alarm
1,722 ratings makes this the most-evidenced alarm here, and false alarms and early failures are what the reports raise.
Current price shown on Amazon. We may earn a commission.
| Power | Hardwired with backup |
|---|---|
| Detects | Smoke and carbon monoxide |
| Watch | False alarms, early failures |
1,722 ratings makes this the most-evidenced alarm here, and the reports are candid: false alarms and units failing within a month both recur.
That is worth weighing rather than dismissing. A hardwired combination alarm is the standard replacement for an expired one, and it is also the format where a nuisance alarm wakes the whole house because the units are interconnected.
Buy it if
- You want the deepest evidence base available
- You are replacing a hardwired unit like for like
Don't buy it if
- You have had nuisance alarms before
False alarms are a safety problem, not an annoyance
An alarm that sounds while you cook is an alarm somebody eventually takes the battery out of. That is the single most common way a home ends up unprotected, and it is a placement problem far more often than a product fault.
Keep alarms at least 10 feet from a cooking appliance. Keep them out of bathrooms, where steam reads as smoke, and away from ceiling fans and supply vents that push air across the sensor.
Photoelectric units tolerate kitchens better than ionisation ones. Where a hallway alarm keeps triggering from cooking, changing the sensor type fixes it more reliably than moving it a foot.
Kidde Smoke and Carbon Monoxide Detector, 30CUDR
739 ratings on a battery combination unit, and owners split on random alarms — which is the failure that gets alarms taken down.
Current price shown on Amazon. We may earn a commission.
| Power | AA battery |
|---|---|
| Detects | Smoke and carbon monoxide |
| Watch | Random alarms reported |
739 ratings, and owners split on reliability with some reporting it going off randomly. On a combination unit that is the report to read hardest.
The app connectivity gets mixed reports too. For a rented home where wiring cannot be altered, a battery combination unit is frequently the only option, and placement is the lever you still control.
Buy it if
- No wiring, and you want both sensors in one unit
- A rented home where you cannot alter wiring
Don't buy it if
- A false alarm at 3am would end your patience
Where they actually go
Inside every bedroom, and in the hallway outside each sleeping area.
On every level, including a basement and excluding an unheated attic.
Ceiling-mounted, at least 4 inches from any wall, because dead air collects in the corner where wall meets ceiling. A wall-mounted alarm goes 4 to 12 inches down from the ceiling.
Not in the kitchen or bathroom, and not within 3 feet of a supply vent.
Carbon monoxide alarms follow the same room logic, and their height matters less than people assume, because CO mixes with air rather than settling.
SC9120B Hardwired Smoke and Carbon Monoxide Alarm
4.6 is the best score here, on a hardwired combination unit that interconnects with others on the same circuit.
Current price shown on Amazon. We may earn a commission.
| Power | Hardwired with backup |
|---|---|
| Detects | Smoke and carbon monoxide |
| Interconnect | Yes, wired |
The highest-rated alarm here at 4.6, hardwired with a battery backup and interconnected, so one unit sounding sets off every unit in the house.
Interconnection is the feature that matters in a multi-storey home, because a basement fire needs to wake a bedroom two floors up. 515 ratings is a modest base for a device this consequential.
Buy it if
- You are replacing one unit in an interconnected chain
- You want the best-rated combination alarm here
Don't buy it if
- There is no existing wiring to connect to
Kidde Smoke Detector, AA Battery-Powered, 20SDR
A smoke-only alarm, which is the right unit for a landing or a loft where carbon monoxide cannot reach.
Current price shown on Amazon. We may earn a commission.
| Detects | Smoke only |
|---|---|
| Power | AA battery |
| Rank here | Lowest rated of 6 |
A smoke-only alarm, and there is nothing wrong with that where no fuel is burned nearby. A landing, a loft conversion or a bedroom in an all-electric home does not need a CO sensor.
4.3 is the lowest score on this page. Buying smoke-only where CO is genuinely a risk is the mistake to avoid here, so check what is in the room before choosing it.
Buy it if
- A room with no fuel-burning appliance anywhere near
- You are covering many rooms cheaply
Don't buy it if
- The room has a boiler, fire or attached garage
The 5-minute audit
Take each alarm down and read the manufacture date. Anything past 10 years goes in the bin today, whatever it sounds like when you press the button.
Then check each one is more than 10 feet from the hob and 4 inches from a wall, and confirm any room with a fuel appliance has a CO sensor rather than a smoke-only unit. Documenting what a fire would cost you is the home inventory that survives a claim, and the papers themselves belong in something rated to UL Class 350.
December adds its own risk: lights or wiring are involved in 40% of home Christmas tree fires, and why Christmas lights start house fires covers the fixes.