Here's the basics:
Prior to 2005, NFPA 720, the carbon monoxide (CO) detection standard set by the National Fire Protection Association, was little more than a recommendation. In 2005, in reaction to greater public awareness of CO poisoning and local CO detection legislation, NFPA 720 was changed from a recommended practice to an installation standard.
In the 2005 edition of NFPA 720, CO detectors were required to be connected to a control panel via a supervisory circuit only. NFPA 720-2009 requires CO alarm signals to be distinct and “descriptively annunciated” from fire alarm, CO supervisory and CO trouble signals. Furthermore, the CO alarm signal should take precedence over supervisory or trouble signals. The actuation of a CO detector or system should be distinctly indicated as a CO alarm signal.
CO detector trouble signals must be indicated visually and audibly at the control panel and supervising station. So the CO detector must have an integral trouble relay that will send trouble conditions to the control panel, such as a sensor failure or sensor end-of-life signal.
UL views the CO sensing element to have a finite life, with most units in the neighborhood of 4-5 years prior to replacement.
