It kind of looks like this. The adjustment tab (lever?) might be metal. I think maybe I'll get a 1 Plus to be safe...
This is a natural gas Reznor garage heater.
a) no, except for a far away wall thermostat that is also connected via the R and W wires. The heater is mounted on a 12-foot high ceiling so human intervention is unlikely as it is only accessible with a ladder.
b) It is connected to a ground conductor coming from the same wire as the mains
c) metallic
d) no
Thanks for the info! I would like to understand more about why this isn't safe so I'll endeavor to research this.
Just to be clear, this is the connection I have currently:
If Shelly 1 Plus is safest for this, I will order one 
I noticed there is mains 115 VAC power being fed into the heater with wire nuts. I pigtailed and connected that to the L and N on the Shelly 1. It seems to be working with my Shelly 1. Is there a reason I need a Shelly Plus 1 instead?
Well, that settles it. The multi-meter measures 27 V AC between the C and R wires. Looks like I need a transformer... or a smart thermostat...
Thanks for the quick reply! I'm confident the WiFi signal is strong. I am going to purchase a multimeter on the way home from work and measure the voltage this evening. Will report back!
I have the Shelly 1 connected this way:
After researching, I realize this is probably wrong, and I think I need to connect it this way:
Thermostat is removed because I really don't need it. The R/W circuit is what switches the heater on, and the C wire provides a return route for the 24 V DC power.
That said, I don't know if this would cause the original issue I was seeing, since the power is connected the same way in both diagrams.
My Shelly 1 won't include in the Shelly app. The Shelly WiFi network appears briefly, but then inclusion is unsuccessful with a timeout error. The WiFi network list in my Android shows the Shelly network briefly, but it seems to disappear. My phone is right next to the Shelly.
The Shelly seems to have power as I can hear it clicking. Sometimes a double click and sometimes a single click, maybe every few seconds.
In case it's helpful, my Shelly is connected to the circuit board of my Reznor heater. The C wire is connected to the Shelly neutral, the R wire is connected to Shelly's line, and the W wire is connected to the Shelly SW. I want to use the Shelly to switch the heater on/off, which is achieved by completing the circuit of the R and W wires.
