Be extra cautious about wireless smoke detectors. Here’s why.
You should classify your devices in separate groups, depending on their criticality. For example, if a motion detector for stray cats on my new lawn doesn’t always trigger the water sprinklers, I can tolerate that. Turning lights on and off in the house, I’d have no tolerance for a button not working when I press it, even if it happens just once per year. And life-critical detectors for smoke, fire or poisonous gases, those not only must work every single time, but be completely immune to perturbances or interferences that may prevent them from working.
HomeMatic IP radio protocol enforces the “duty cycle” limit for each device. This is the legal regulation that any device transmitting in the free band of 868MHz is allowed a broadcast time of maximum 1% per hour. So if you have for example a motion detector in a high traffic area, it will keep transmitting “movement detected” until it has added up 36 seconds of transmission time in the past 3600 seconds, then will stop transmitting for the rest of the hour.
If that motion sensor is among the low critical devices, where the lack of functionality is not going to cause a significant issue, that’s fine. But would you risk your life because your choice for home automation’s protocols stop working on purpose? If your other HMIP components are causing your CCU to frequently hit 100% of that duty cycle, the CCU will receive the alarm from one of the smoke sensors but will not be able to transmit the trigger message to other sensors, evacuation lights, or a siren.
Ideally, the smoke sensors would be part of a completely independent, wired alarm system. If you must go wireless, pick a technology that does not overlap with anything else in your surroundings and does not depend on any device outside of the enclosed system. I’ve seen people use WiFi smoke detectors - what if your power goes out (perhaps related to the fire) and your WiFi network is down? What if you live in a high density condominium where there are 60 WiFi networks from all neighbors and the WiFi collisions are so frequent that most of your devices are repeatedly knocked off the network and continuously either reconnecting or retransmitting packets?
I used the wireless system from Ei Electronics, an Irish company specializing in this kind of equipment. Specifically, I have installed 11 smoke detectors model Ei605CRF-3XDR and one output interface Ei428-D. They work with regular 9V batteries. The wireless module (optional, if you don’t want to use the wired interconnection) also uses the same 9V battery for power, unlike most other radio smoke detectors whose radio module have a soldered-on battery lasting 3-5 years then you have to throw out the radio module as well and buy a new one. So when the smoke detectors need to be replaced in 10 years, I no longer have to spend extra on the radio modules. All devices create a radio “mesh” communicating one with the others directly. There is no need for any central system or cloud-based service, or existing WiFi network or anything like that. There was an extra status and control panel I could install as well, but I chose not to. The only limitation is that the consumer-grade configuration allows maximum 12 (or 13?) devices in the mesh, including sensors and interfaces and control panels etc.; if you want more, you need to get in touch with them for instructions.
The interface acts as a simple switch with both normally open and normally closed positions. From there you can hook it up to OpenHAB through any means you like - perhaps one of the GPIO pins on a Raspberry Pi. Mine is connected to a KNX interface, so that the smoke sensors automatically trigger the fire alarm over KNX, raising all blinds and turning on all lights in the stairwell; OpenHAB is only indirectly notified through the KNX interface, and is not involved in controlling anything.
Costs: each smoke detector including the separate radio module was €38,81, and the interface was €84,95, all including VAT and free shipping. Ordered from elv.de in January 2019. The smoke detectors without radio module are €18,95 each, for when I need to replace them in 6 years. Right now I can no longer find this model (with R at the end, including the battery-free radio module), and the separate radio modules with built-in battery are like €40 each.
