FYI.
I was recently contracted by a client who wanted smart control of his switches, but wanted his existing switches to remain. The solution, is to use "in wall relays".
However, given, that the house is relatively older, the internal junction boxes ("gangs"), are relatively small. Here I present the actual sizes of the three smallest dual channel devices that I could find.
(Note: There are significant differences between these, but if size is your key parameter, this should help you decide.)
Type
Protocol
Size
Aeotec Dual Nano Switch
Zwave +
4.318 x 4.064 x 2.032
Shelly 2.5 UL
WiFi
4.2 x 3.6 x 1.4
EnerWave
Zwave +
5.156 x 4.369 x 1.575
Quibino Flush 2
Zwave +
4.06 x 3.56 x 1.52
Fibaro FGS 223
Zwave +
4.24 x 3.81 x 2.032
RGB Genie ZW-4001
Zwave +
4.55 x 5.50 x 2.03
Sonoff ZBMini
Zigbee
4.26 x 4.26 x 2.0
RGB Genie ZB-4002
Zigbee
4.55 x 4.50 x 2.03
Zooz Zen52
Zwave +
3.81 x 3.6 x 1.8
Sonoff Mini R4 (Extreme)
WiFi
4.00 x 3.3 x1.7
(updated on 2022-12-14)
If anyone is aware of another dual channel device that is comparable, please let me know, and I amend this chart.
Please note: For our European friends, (from what I understand), size is not an issue, as there is plenty of room inside the junction box. Also, the newer, North American plastic boxes are also much larger.
They are the same size though height may vary slightly. Even the Flush 1D dry contact relay is the same size.
IMO the Qubinos are not my favorite devices. They are pain in rear to exclude if you ever have to reset them and the dual relay was spamming my zwave network causing problems. But they are very small.
The reason that I used dual instead of single relays is because with old metal boxes, I just couldn't fit a relay controller in!
Maybe a professional electrician could do it, but I certainly can't.
If you happen to have a neutral wire, then you should install the version that requires neutral. This means that it could be used as a switch not only as a dimmer.