Why a passing test isn't enough
The Consumer Product Safety Commission notes that a CO alarm's test button checks the circuitry, not whether the sensor still detects carbon monoxide accurately. Sensors age, which is why both kinds of alarm have a replacement date. Keep testing monthly, but treat the date on the unit as the real deadline.
How to find the date
Take the alarm off its mounting plate and look at the back. Smoke alarms carry a manufacture date, and the 10 years count from that date, not from when you bought or installed it. For CO alarms, CPSC says expiration dates vary by model; check the label and manual, and note that many newer units have an end-of-life signal. Write the replace-by year on the outside of each unit with a marker so the next check takes seconds.
If an alarm has no readable date at all, assume it's past due.
What to do by alarm type
- Smoke alarm with a replaceable 9-volt battery: change the battery once a year and replace the whole alarm at 10 years.
- Smoke alarm with a sealed 10-year lithium battery: there's no battery to change, so replace the whole unit on the manufacturer's schedule.
- Hardwired smoke alarm: change the backup battery once a year and replace the alarm at 10 years. The U.S. Fire Administration says hardwired alarms should be installed by a qualified electrician, so don't swap one yourself if it means opening wiring. See when electrical work in Philadelphia needs an L&I permit.
- CO alarm or smoke and CO combination alarm: follow the manufacturer's date or end-of-life signal. Plug-in and hardwired CO alarms should have a battery backup.
Buying the replacement
Check the box before you buy. CPSC says CO alarms should meet UL 2034, and Pennsylvania's carbon monoxide law accepts combination units that meet both UL 2034 and UL 217 and give distinct warnings for CO and smoke. USFA recommends interconnected smoke alarms, so one going off sets off the rest, and either dual-sensor alarms or a mix of ionization and photoelectric types. USFA also points out that strobe-light and bed-shaker alarms exist for people who are deaf or hard of hearing.
When you replace one, it's a good time to check coverage. USFA calls for smoke alarms inside and outside each bedroom and on every level, including the basement. Philadelphia's emergency management office adds the top of open stairways and the bottom of enclosed ones, and recommends 10-year sealed lithium alarms. For CO, EPA lists every livable level including the basement, near sleeping areas, at least 15 feet from fuel-burning appliances, and near the door to an attached garage. CPSC adds not to put a CO alarm in a kitchen or right above a fuel-burning appliance.
Philadelphia, Bucks and Montgomery County specifics
- Free installs in Philadelphia: owner-occupied one- and two-family homes can request free smoke alarm installation from the Fire Department through Philly311, by app or phone. The city says it can take up to 90 days, so don't leave an expired alarm up while you wait; a basic alarm is inexpensive, and USFA lists them from about $6. City page.
- Renting in Philadelphia: the city says landlords must provide smoke alarms on every level, and tenants can report a violation through 311.
- Selling anywhere in Pennsylvania: Act 121 of 2013 requires the seller to disclose on the property disclosure statement whether CO detectors are installed. Owners of multifamily rentals with fossil-fuel heat or appliances, a fireplace or an attached garage must provide approved CO alarms. Act 121 text.
- Bucks and Montgomery County townships: the state law lets municipalities set stricter rules, and USFA says to check with your local fire marshal about which alarm types are required. CPSC gives the same advice for CO alarms through your local building code official. Ask before you sell or rent out a home.
If a CO alarm goes off during the swap
Don't assume it's just the old unit failing. CPSC says to get outside to fresh air, call 911, and stay out until responders say it's safe. Our guide on when a carbon monoxide alarm means leave the house covers how to tell a full alarm from a short chirp.