Reading the Manufacture Date and the Ten-Year Replacement Rule

Why this matters

A smoke alarm with a green light and a working test button can still be past the point where anyone should trust it. The test button proves the horn and the circuit; it does not prove the sensing chamber still responds to smoke the way it did when new. That is why the replacement rule runs on a calendar, not on a test, and the calendar starts at a date printed on the back of the alarm that almost nobody reads. A tech who reads it, writes it down and tells the customer has done something a test button cannot. A tech who reads the "installed" sticker instead can be off by years in the wrong direction.

The rule, and whose rule it is

Smoke alarms. NFPA 72, in the edition your jurisdiction adopts, calls for household smoke alarms to be replaced no later than 10 years from their date of manufacture, and sooner if they fail to respond to a test. NFPA 72 binds a homeowner through its adoption by reference in the building or fire code, and the alarm maker's instructions say the same thing, so in practice the 10-year rule reaches almost every house. Inspectors on permitted work check the date.

CO alarms. There is no single code number of years. A CO alarm is replaced at the end of the service life its maker states, or when its end-of-life signal sounds, whichever comes first. Stated lives on current units commonly run 7 to 10 years; the label is the authority, not the common range (see related: The CO Alarm End-of-Life Signal and How Long a Sensor Lasts).

Combination smoke and CO alarms. Replaced on the earlier of the two clocks: 10 years from manufacture for the smoke side, or the stated CO life if that is shorter.

Heat alarms. Follow the maker's stated replacement interval; the label is again the authority (see related: Heat Alarms Where a Smoke Alarm Would Lie).

Why the install date is not the clock

An alarm can sit in a warehouse, on a store shelf or in a contractor's van for a year or two before it goes on a ceiling. The 10-year rule counts from manufacture because that is the one date the device itself carries and nobody can lose. An "installed" sticker, a date written in marker, or the homeowner's memory of when they moved in are all dates about the house, not about the alarm. They also run in the flattering direction: every one of them is later than the manufacture date, so trusting them makes an old alarm look younger.

The same trap catches sealed long-life units, many of which count their battery life from activation. The battery may keep going past the label date; the rule does not.

What the back of an alarm looks like

Here is the kind of thing you will find on the back of a household smoke alarm and a plug-in CO alarm. Wording and layout vary by maker; the fields are what to look for.

+------------------------------------------+
| SMOKE ALARM   photoelectric              |
| listed to UL 217                         |
| 120 VAC 60 Hz   backup: 9 V battery      |
| DATE OF MANUFACTURE: 2015 JUN 09         |
| replace 10 years after date of mfg       |
+------------------------------------------+
| (sticker, added later by an installer)   |
| INSTALLED 2018                           |
+------------------------------------------+

+------------------------------------------+
| CO ALARM   listed to UL 2034             |
| battery: 2 x AA   plug-in 120 VAC        |
| MFG DATE: 2019-03-14                     |
| REPLACE BY: 2026-03                      |
| date installed: ____________             |
+------------------------------------------+

Read it in this order. The listing line tells you what kind of device it is: UL 217 for smoke, UL 2034 for CO, both for a combination unit. The manufacture date is the clock. A replace-by date, where the maker prints one, governs if it is earlier than 10 years from manufacture. The installed line is useful for the job history and nothing else.

Some makers print the date as a code (a year-and-week number, for example) rather than in words. The maker's instructions or support line decodes it. A date you cannot decode is treated the same as no date.

No date, or no readable label: replace. An alarm old enough to have lost its label is old enough to replace.

Reading it without making things worse

To read the back you have to take the alarm off its base, usually by twisting it a quarter turn. Two hazards come with that.

Height. Use a stepladder set fully open and locked, not a chair or a bucket, and while both hands are on the alarm, keep your body centered between the rails and stay off the top two steps.

Live wiring on hardwired units. On a hardwired alarm, the base and its plug-in harness stay energized when the alarm comes off. Unplug the harness from the alarm only if you need to and only by the connector body; do not touch the conductors, the wire nuts or the box, and do not remove the base. If the base, the harness or the box is loose, damaged or has exposed conductors, stop, leave it as found, and note it for an electrician.

Every alarm you take down goes back up and is tested before you move on, so the house is never quieter than you found it for longer than it takes to read a label (see related: The Smoke and CO Alarm Disturb and Restore SOP). On an interconnected system, removing one alarm can leave a gap in the chain while it is off, which is another reason to read one at a time.

Worked example: seven devices, three overdue

A tech doing an end-of-job check on October 21, 2026, in a two-story house with five smoke alarms and two CO alarms, reads every label. The results, with the arithmetic shown:

Check the arithmetic. Hall smoke: June 2015 plus 10 years is June 2025; June 2025 to October 2026 is 12 + 4 = 16 months. Hall CO: March 2026 to October 2026 is 7 months. Bedroom 1: October 2026 to November 2027 is 12 + 1 = 13 months.

What the sticker would have said. The hall smoke alarm carries an "Installed 2018" sticker. Read from the sticker, the alarm is 8 years old and has 2 years left. Read from the label, it is 11 years 4 months old (June 2015 to October 2026) and past due. The two readings differ by 3 years 4 months: the time the alarm spent between the factory and the ceiling, plus whatever the year-only sticker rounded away.

What the hall CO alarm shows. It is 7 years 7 months old (March 2019 to October 2026), well inside what people imagine a CO alarm lasts, and still overdue, because its maker set a 7-year life. Without the label read, it looks fine; its end-of-life signal may not have started yet, or it may have been silenced.

Three overdue: hall smoke, bedroom 2 smoke, hall CO. These go to the customer in writing with the dates. Bedroom 1 goes on the note as "next due November 2027," so the shop can remind the customer. The hall and bedroom 2 smoke alarms are on a wired interconnect with the other smoke alarms, so their replacements have to be compatible with the units staying up (see related: Hardwired, Interconnected and Battery Backup: What Each Word Means). Hardwired replacements are electrician work where your state or locality licenses it, as many do; a plug-in CO alarm is not.

What changes the answer. If the hall CO alarm's label had said "replace 10 years after date of manufacture," its replace-by would be March 2029, and it would be OK. The rule did not change; the maker's stated life did. If bedroom 2's alarm had a date code the tech could not read on the spot, the right call is still "treat as due" on the note, with "date code unreadable" beside it, so a later visit can decode it rather than assume it.

One clock or several

Replacing only the overdue units is correct and leaves the house on staggered clocks, so somebody has to come back every year or two. A reasonable default to offer: when more than half of the smoke alarms in a house are within 2 years of their replace-by date, recommend replacing the whole set at once, so the house runs on one date and stays interconnect-compatible. In the example above it triggers: of five smoke alarms, two are already past due and one is due in 13 months, so three of five are past or within 2 years of their date, which is more than half. Offer it, priced as one visit, and let the customer choose. A warranty replacement or a unit swapped after a failed test carries its own new manufacture date; record it, because it has just become the youngest clock in the house.

What to record, per device

Location, device type and listing, manufacture date as printed, replace-by date (calculated or printed, and say which), test result, and what the customer was told. Record the date exactly as the label gives it, not rounded to the year, because a year's rounding is the difference between "overdue" and "fine" for an alarm made in the right month. In a field-service app, a recurring reminder set to the earliest replace-by date in the house is the cheapest repeat visit the shop will ever book, and it is one the customer is glad to get.

References

• NFPA 72, National Fire Alarm and Signaling Code, in the edition your jurisdiction adopts: replacement of household smoke alarms at 10 years from date of manufacture
• UL 217 (smoke alarms) and UL 2034 (CO alarms), the listing standards named on the label
• Alarm manufacturer instructions, for stated service life and date-code decoding
• See related: The Smoke and CO Alarm Disturb and Restore SOP; The CO Alarm End-of-Life Signal and How Long a Sensor Lasts; Ten-Year Sealed Battery Alarms and What They Change