Smoke and CO alarms produce different types of signals to trigger an alert. Smoke alarms and heat alarms send a constant 9VDC signal on the interconnect wire, while CO alarms send a pulsing 9VDC signal. Smart Interconnect is a feature that allows the detector to interpret both types of signals for smoke and CO on a single interconnect wire or over the wireless interconnect signal, allowing all detectors in your home to sound during both smoke alarms and CO alarms.
Although there is no electrical risk associated with having older alarms connected via the interconnect wire, devices that do not support Smart Interconnect will function as follows:
When smoke is detected by a smoke alarm or a combination smoke and CO detector, all interconnected smoke, heat, and Smart Interconnect CO alarms will sound the smoke horn pattern. CO alarms that do not have Smart Interconnect will remain silent.
When CO is detected by a CO alarm or a combination smoke and CO detector, all interconnected CO alarms and Smart Interconnect smoke alarms will sound the CO horn pattern. Smoke and heat alarms that do not have Smart Interconnect will remain silent.
*Smoke and CO hazards require opposite responses, so NFPA Standard 72 (Section 29.11.2) specifies that alarms without Smart Interconnect must remain silent, ensuring that only the devices emitting the correct horn pattern are heard. The standard states that interconnected alarms must respond appropriately to the specific hazard detected, either by sounding the correct pattern or staying silent if they cannot.