When to Replace Smoke Detectors (and How Often to Test Them)

Smoke detectors have an expiration date. Most homeowners don't know this, and most inspections don't catch it. The sensor inside a smoke detector degrades over time, and after 10 years, the National Fire Protection Association (NFPA) states it can no longer reliably detect smoke. A detector that passes the monthly test by making a sound when you press the button might still fail to alert you in an actual fire — because the sound comes from the battery, not the sensor.

When you move into a home, the first task is finding out how old the smoke and CO detectors are.

How to Find the Age of Your Smoke Detectors

Every smoke detector manufactured since the late 1980s has a manufacture date printed on the back or inside panel. Remove the detector from its ceiling or wall bracket and check the back of the unit. You'll see either:

• A printed year (e.g., "Manufactured 2016")
• A date code (e.g., "2015-04" indicating April 2015)

If the detector is 10 years old or older, replace it immediately, regardless of whether it still beeps when tested.

If you're moving into an existing home and can't find a date, the safest assumption is to replace all detectors. A 3-pack of combination smoke/CO detectors costs $60 to $80 — far less than the risk of having expired units.

How Often to Test Smoke and CO Detectors

Monthly: Press and hold the test button until the alarm sounds. Release and verify it stops. If the detector doesn't sound, check the battery first. If a fresh battery doesn't fix it, the unit is failing and needs replacement.

While testing, visually inspect the housing for:

• Dust or cobwebs covering the sensor opening (can block smoke from reaching the sensor)
• Discoloration or scorch marks on the housing (rare, but indicates a previous fire event)
• Insects — spiders in particular build webs inside detector chambers and trigger false alarms

Annually: Replace batteries in all non-sealed detectors. Some newer units have a 10-year sealed battery designed to last for the full life of the unit — these don't need annual battery replacement, but do need to be replaced after 10 years.

Every 10 years: Replace the entire smoke detector unit, regardless of whether it's still functional.

Every 5 to 7 years: Replace CO detectors. Carbon monoxide sensors degrade faster than smoke sensors. The manufacture date on the back panel applies.

Where Smoke Detectors Are Required

Code requirements vary by jurisdiction, but best practice (and the NFPA 72 standard used across the US, Canada, and referenced in UK and Australian standards) is:

• One detector inside each bedroom
• One detector outside each sleeping area (hallway)
• One detector on each level of the home, including the basement
• One detector at the top of each staircase between floors

Where not to install: In kitchens, directly next to bathrooms (steam from showers triggers false alarms), or in garages (vehicle exhaust will constantly trigger CO detectors).

In the UK: Building regulations require at minimum one smoke alarm on each storey and one heat alarm in the kitchen. Scotland requires interlinked smoke, heat, and CO alarms — all mains-powered with battery backup — in all homes as of 2022.

In Australia: Requirements vary by state. Victoria and Queensland require mains-powered photoelectric smoke alarms in all bedrooms and interconnected throughout the home. Check your state's specific requirements.

Ionization vs. Photoelectric: Which Type to Buy

Ionization detectors: Faster to detect flaming fires with large particles. Slower to detect slow, smoldering fires.

Photoelectric detectors: Faster to detect slow, smoldering fires. Slightly slower on fast-flaming fires.

Best option: A combination unit with both ionization and photoelectric sensors, or a photoelectric detector with a CO sensor. Most home fires start as slow smolders (cooking, electrical faults, cigarettes) before transitioning to open flames. A photoelectric-only or combination detector provides more reliable early detection in these more common scenarios.

Major brands (First Alert, Kidde) offer reliable combination units at $20 to $40 per detector.

Carbon Monoxide Detectors: What You Need to Know

Carbon monoxide is a colorless, odorless gas produced by fuel-burning appliances — gas furnaces, water heaters, dryers, ovens, fireplaces, and vehicles. Exposure to moderate concentrations causes headache, dizziness, and confusion within hours. High concentrations cause rapid incapacitation and death.

Every home with any gas-burning appliances, a gas furnace, or an attached garage needs CO detectors.

Placement: Within 15 feet of each sleeping area. Ground level is fine — CO mixes with air rather than settling low or rising high, so height is less critical than placement near bedrooms where people sleep.

Replace every 5 to 7 years. CO sensors contain electrochemical cells that deplete over time. A CO detector that still sounds when tested may no longer accurately detect CO concentrations.

Important: If a CO alarm sounds and it's not a test:

1. Leave the house immediately with all occupants
2. Do not re-enter
3. Call emergency services (911 in US/CA, 999 in UK, 000 in AU)
4. Let the fire department verify the source before returning

The source is almost always a malfunctioning furnace, water heater, or blocked flue vent — all of which require professional repair before the home is safe to reoccupy.

The First-Year Homeowner Maintenance Calendar includes monthly test reminders and the annual battery replacement task for every detector in the home. It also flags the 10-year and 5-year replacement windows so you're never in a home with expired life-safety equipment without realizing it.