Four beeps and a pause (carbon monoxide alarm)

How to read it

The pattern is called temporal-4, or T4. Each beep lasts a tenth of a second, with a tenth of a second between beeps, and after the fourth there are 5 seconds of silence. NFPA 720, the carbon monoxide standard, allowed each part 10 percent either way in its 2015 edition. After the first 4 minutes the 5-second pause may stretch to 60 seconds, and the signal repeats until the alarm resets or someone silences it. UL describes it for code officials as four spaced tones followed by a five-second silent period. NFPA’s Fire Prevention Week tip sheet says four loud beeps mean carbon monoxide is present in the home. Each beep is a fifth as long as a smoke alarm’s, and UL 217 requires a combined smoke and CO alarm to sound the three-beep pattern when it detects smoke.

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From a building’s fire alarm system

Carbon monoxide detection on a building’s fire alarm system can sound the same rhythm through its horns. Johnson Controls’ data sheet for its Autocall CO module says CO warnings differ from fire alarm warnings and that the temporal-4 pattern tells responders which condition it is. Potter’s low-frequency horn sounds temporal-4 when wired to Potter’s synchronisation module.

NFPA 720 and NFPA 72

NFPA 720 set the rules for carbon monoxide alarms from 2003. In 2018 NFPA said it would withdraw the standard once the 2019 edition of NFPA 72, the fire alarm code, was issued, with NFPA 720’s requirements moved into NFPA 72, Occupational Health & Safety reported. Manuals for alarms of that era, such as Gentex’s CO1209, quote the timing from NFPA 720’s 2015 edition.

The pitch of the beeps

UL describes the CO signal by its rhythm alone, and no CO alarm maker we found publishes its horn’s frequency. The clip on this page uses 3.2 kHz, the frequency CPSC staff measured on a residential smoke alarm’s horn, inside the 3,000 to 4,000 Hz they give as typical for smoke alarms. A real alarm may sound higher or lower; the rhythm is what identifies it.