CO and Smoke Alarms: Where They Go, How Often to Test

Two devices and one habit cover the whole problem. A carbon monoxide (CO) alarm on every level at 1.5 m, a smoke alarm on each ceiling 30 cm out from the wall, a 30-second test once a month, and 15 minutes of cross-flow ventilation twice a day. Get those 4 things right and a closed-window winter stops being a risk you manage by smell.

Cats make this urgent in a way that dogs do not. A 4 kg cat has roughly one-eighteenth the body mass of an adult human, sits closer to the floor where some gases pool, and hides when it feels ill, so the alarm is often the first signal anyone gets.

Why the cat goes down before you do

CO binds hemoglobin about 200-250 times more tightly than oxygen does, and a small body reaches a dangerous saturation faster at the same room concentration. A person may notice a headache at a level that already has a cat ataxic and hiding under a bed. Cats also spend 14-18 hours a day asleep in enclosed spaces: a cupboard, a carrier, the gap behind the sofa.

Smoke is the same story with a shorter clock. Most fire deaths are from smoke, not flame, and a room can go from clear to untenable in 2-3 minutes. A cat that hides instead of fleeing uses up most of that window.

CO alarm placement: 1.5 m, one per level

Mount the CO alarm at 1.5-1.8 m above the floor on a wall you can see from the doorway. CO mixes fairly evenly with room air, so height matters less than proximity to the source and to where people and cats actually are; 1.5 m is the height most manufacturers specify and the height at which you will hear it.

Keep it 1.5-3 m from a gas stove, boiler or water heater, and not directly above one, where hot exhaust dilutes the sample. Do not put it inside a closed cupboard, in a dead-air corner, or within 30 cm of a ceiling. One unit per floor is the minimum; add one in every room with a fuel-burning appliance and one outside each sleeping area.

Smoke alarm placement: ceiling, 30 cm out

Smoke rises, so the ceiling is right, but not the corner. Fit the unit at least 30 cm from any wall and 60 cm from a light fitting or air-conditioner vent, because the still air in a corner delays response by several minutes. On a pitched ceiling, stay 30-90 cm below the apex.

One alarm covers roughly 40-60 square metres of normal flat ceiling, which is most apartments' entire living area, so 2 units per flat is the practical minimum: one in the hallway outside the bedroom, one in the living room. Interconnected units (wireless, 60-150 RMB each) all sound together, which matters when the cat is asleep in the far room.

Placement and service table

Device	Height	Distance from source	Units per 90 m2	Test	Replace	Cost (RMB)
CO alarm, electrochemical	1.5-1.8 m	1.5-3 m from stove	2	Monthly, 30 s	5-7 years	80-220
Smoke alarm, photoelectric	Ceiling, 30 cm off wall	60 cm from vents	2	Monthly, 30 s	10 years	40-120
Combined CO and smoke	Ceiling, 30 cm off wall	1.5 m from cooker	2	Monthly, 30 s	7-10 years	120-300
Gas leak alarm, natural gas	30 cm below ceiling	1 m from meter	1	Monthly, 30 s	5 years	70-180
Heat alarm, kitchen	Ceiling, 50 cm off wall	1 m from hob	1	Monthly, 30 s	10 years	60-150

The kitchen row is not decoration. A photoelectric smoke alarm 1 m from a wok will sound 3-4 times a month, someone will take the battery out in week 3, and the flat ends up with no working alarm at all. A heat alarm in the kitchen plus a smoke alarm in the hallway keeps both jobs.

Test monthly, and mean it

Press the test button for 30 seconds on the first day of each month; put it in the phone calendar with an alarm, because nobody remembers. The button only proves the sounder and the circuit, not the sensor, which is why the replacement date on the label is the number that actually saves lives.

Write the install date on the back of each unit in marker. Electrochemical CO sensors degrade quietly after 5-7 years and often fail without a fault signal; photoelectric smoke chambers clog with dust and cooking grease and get 2-3 times less sensitive in 5 years in a kitchen-adjacent hallway. Vacuum the vents every 3 months with a brush attachment.

Battery-only units need a battery change every 12 months.

Ventilation that actually moves air

Crack 2 windows on opposite sides of the flat for 10-15 minutes, twice a day, ideally after cooking and before bed. That gives roughly 1 full air change in a 70 square metre flat and drops indoor CO2 from 1,500-2,000 ppm toward 600-800 ppm. One window open 2 cm for 8 hours does far less than 2 windows open wide for 12 minutes.

Run the range hood for 10 minutes after the gas goes off, not just while cooking, and check that it vents outdoors rather than recirculating. Keep indoor humidity at 40-60 percent; below 30 percent the cat's airways dry out, above 65 percent mould and dust mites take over. Never run a charcoal grill, a generator or a portable fuel heater indoors, and never idle a car in an attached garage, even with the door open — CO travels through the door gap in minutes. Alarm and filter prices are compared in the cost section, and the drinking-water trade-offs in water fountains.

If the alarm sounds and the cat is inside

Get out first, then call from outside. Do not spend 3 minutes hunting for the cat in a smoke-filled room; visibility at head height can drop to under 1 m in 2 minutes, and a person who goes back in becomes the second casualty. Leave a door open on the way out if it costs you nothing, and tell the fire crew the cat's usual hiding places: under the bed, in the wardrobe, behind the boiler.

After a real CO event, every cat in the household needs a vet exam the same day even if it seems normal. Signs that come later include lethargy, refusal to eat, wobbling, and bright red gums, which are a late sign rather than an early one.

Five steps this weekend

Count your alarms. Fewer than 1 CO alarm per level means a hardware run today.
Move any alarm that sits within 1 m of a stove or inside a cupboard.
Press every test button for 30 seconds and note which ones stay silent.
Write install dates on the back and set a monthly calendar reminder.
Open 2 opposite windows for 12 minutes tonight, and again after tomorrow's cooking.

Frequently asked

Can I skip the CO alarm if I have a smoke alarm?

No. They detect different things and one will not cover the other. A smoke alarm responds to particles, so it stays silent during a slow CO leak from a boiler or a blocked flue, which is the classic winter death. You need at least 1 CO alarm per level plus 1 smoke alarm per 40-60 square metres of ceiling.

How often should I actually test the alarm with a cat in the house?

Once a month, 30 seconds per unit, and do it while the cat is in another room with the door shut. A 4 kg cat hears a 85 dB sounder at close range as painful and may bolt into a cupboard for hours. Test, then air the room for 10 minutes before letting the cat back in.

Is 15 minutes of ventilation a day enough for a flat with 2 cats and a gas stove?

Twice a day for 10-15 minutes with cross-flow is the practical minimum, plus 10 minutes of range hood after every cooking session. If the windows are single-glazed and the flat is under 50 square metres, add a third session before bed. Humidity is the check: 40-60 percent means the routine is working.