Zwave switches in metal gang boxes
Zwave signals are Omnidirectional (360 degrees) so as you noted, you can catch the signal in just one direction as long as you have another device to do so. And of course you want to avoid a metal faceplate since then you’ve closed off all four sides.
OK, so you’ve got a Z wave radio inside a metal box with a plastic face plate. Now you have to get signal through the plastic, that’s the critical point. So you need another mains powered Z wave device (not battery powered, those don’t repeat) within about 40 feet (you can go longer if it’s a zwave plus device) where the signal can get directly to the plastic faceplate. It could be a Z wave bulb on the ceiling, it could be a pocket socket on the opposite wall, maybe even a pocket socket in the next room over. (Pocket-sockets have the advantage that they themselves are not trapped in a metal box.)
Also be aware that when you have metal gang boxes, that means you have a lot of metal inside your walls, even if those aren’t networked devices. So there could be dead spots as the signal tries to go through walls as well.
So most of the time if you’re having trouble with signal to zwave light switches in metal boxes, the following should help:
One) choose Z wave plus over classic zwave if you have a choice. You’ll get much better range.
Two) choose a plastic face plate for the switch. Not ceramic, not metal.
Three) put a Z wave plus pocket socket within 75 feet of the light switch so that the signal could go directly to the faceplate. ( it’s OK if the line is through a wall, but then the range will be reduced somewhat.)
One pocket socket can repeat signal to all of the switches in a room although it will be least successful with the ones on the same wall that it is on, because then there won’t be a direct line through the plastic.
There are many different factors that go into pairing range, which is why there’s so much confusion about this.
First, the easiest. If you have zwave plus light switches, as long as the switch is within one hop of a repeater, you can pair it in place. So, easy.
If it’s a Z wave classic light switch, however, it has to be within one hop of the hub or a minimote at the time of initial pairing or it can’t join the network. For light switch, you can install it where you want it and then bring the hub to the switch, or bring a minimote to the switch.
Z wave locks have still yet different pairing rules, as they must be paired within “whisper distance” in order to exchange a security key with the hub. But that doesn’t apply to light switches.
After you have all the devices where you want them to be, it’s a good idea to run a Z wave repair just to make sure that all the neighbor tables are up-to-date.
OK, what all this means is once the switch has been paired to the network and the neighbor tables are up-to-date, you get much longer range because the switch can pass messages to other mains powered Z wave devices like a relay race and that’s what extends the total distance that your devices can be from the hub. Each device still has the same signal strength and sends a message for the same number of feet. The difference is just that that message can be picked up by a device other than the hub and passed along until it eventually reaches the hub.
I definitely would stick with code requirements, but as long as you’re willing to put a pocket socket in the right place and use plastic faceplates you should be fine even with a metal gang box.