Having mains powered interconnected smoke alarms is now the standard and any new home built in Canada will come with smoke detectors installed in this manner. This includes one interconnected smoke detector in each bedroom. A large home may have many interconnected smoke detectors. This is building code in Canada. Many people choose to install smoke detectors that are also Carbon Monoxide detectors, this takes care of two monitoring requirements with one device.
Some people (myself included) have upgraded their older home to this new building code standard. Kiddie and First Alert are two of the major manufacturers of these detectors, they both supply relay kits for their mains powered interconnected smoke alarms. One manufacturer states the purpose of these relays are to: "activate auxiliary warning devices such as external bells and sirens, hallway or stairway lighting". Obviously an individual with a HE is simply concerned about the relay notifying HE of the smoke/CO alarm. I do this by connecting the relay to a Zooz Zen16, but it can be done by connecting the relay to any contact sensor, or using an hubduino solution.
Installing a manufacturer supplied relay on the interconnect wiring of your smoke alarms does NOT violate any code. Building inspectors take no issue with it, since it in no way disables its standalone functionality.
People considering going this route should be aware that First Alert only makes one relay that will notify you when either a smoke or carbon dioxide condition is present. With this First Alert relay you have no way of easily determining whether the alarm is due to smoke or carbon monoxide (I believe there is an hubduino solution to this, but it is not straightforward). With Kiddie, there are two relays that you can install on your interconnect wiring. One relay will notify you of the smoke alarm and the other will notify you of the CO alarm.
In my opinion, having these relays are the best way to notify your HE of a smoke or CO alarm. But each to their own.
