Where is the fireplace transformer relative to the switch? Do you have access to the fireplace controls through an inside cabinet access panel?
You may be on to something here. I did not consider putting the Zen51 in the space where the fireplace hardware is behind the fireplace front, lower grill. I must take a look.
If you have a low voltage transformer you can access I would get the zen58 and wire it up for power and that wall switch at that location. Meaning you can put the zen58 by the fireplace controls where the transformer is located. I assume the wall switch is a low voltage switch.
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This is part of the electronics in my fireplace. I can connect the Zooz Zen51 to the Hot/Neutral contacts for 120V and connect the IN and OUT wires to my original low voltage switch. Correct?
I'd want you to hire a professional.
I believe the zen51 requires 120v on the switch.
If your switch is low voltage you can get the zen58 and power it off that junction box with a transformer. Or.. The zen57 which is line powered with dry contact relay and no voltage on the switch.. If I read the specs correctly.
That is exactly what i have in my fireplace. I would still suggest the zen 16 or zen 17.
What kind of fireplace is it. Do you have a control module that controls the fireplace
With my setup the fireplace control module has a wiring harness that has two wires intended to go to a low voltage switch. The builder installed a wire pair to go to a low voltage switch. The builder used a some crimped wire connectors between the controller wires and the switch.
I simply created two pairs of jumper wires and use them to connect the zen 17 between the controller and the switch.
My concern with the zen51 is that it has exposed pigtails and as such maybe not the best option for that environment under the fireplace.
It sounds like you have the same as I do. I have the Heat & Glo 8000C. My two wires go to a standard light switch that is low voltage. I have the smart switch shown above installed over that switch that flips the switch on and off remotely. But it does not work consistently and it drains batteries fairly quickly. That's the reason I want to change to line voltage.
How is that?
Pretty sure that zen51 can't do low voltage switch. It needs 120v on the input for the switch from my reading.
Again the zen57 makes the most sense. The exposed pigtail on it has no voltage. That junction allows for insulated connectors. You can tape or shrink wrap the zen57 or put it in a junction box. I don't know for sure but that whole space must be treated like the interior of an appliance because those junction boxes have exposed connectors.
The link I posted far up thread for zooz kb is older and talks about the zen17 use with fireplaces.
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To expand on that. This version of the Zen58 only has pigtails for the switch.
You still need a way to power it though.
It does sound like we have similar setups.
My requirement was ease of removal and not changing anything that was installed with the Fireplace. I like the Zen 17 because if i choose to add a aux fan for heating improvements i can hook it to the other relay in the Zen 17 and have independent control of the fan. Wether it is a zen17 or zen 58 they still need some kind of ac to dc adapter to power the zooz device.
Looking at the wiring diagrams I don't think the zen 51 is even a option. It looks like it is passing power and not just opeming closing a relay.
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Let's recap. Zen51 has a dry contact relay, but the switch needs 120v. Zen58 is powered by low voltage, has a dry contact relay and has a no voltage switch pigtail. Zen57 is the same as Zen58 accept it's powered by line voltage or 120v in the US. So in this case if the op didn't want a low voltage wall wart plugged into the junction box posted up thread they could wire the line voltage into a Zen57 from the two rails on the outside of that junction box. Then connect the leads currently going to that switch to the Zen57 and then connect the switch pigtail on the Zen57 to the leads coming from the switch.
You are making it more complicsted then it needs to be. The op also mentioned he has the same setup i do. In th a t setup when you take the fireplace cover off there is a lower section with a control box and 3 plugs. One plug is likely used by a dc power adapter for the control box. The op can simply connect whatever device he is going to use down there instead of messing with anything in the cubby area were the switch and a wall wart is a issue.
If we are certain the zen 57 can be wired with no power on the in out connectors then it will work fine. Just be sure however it is wired nothing else is putting power on that circuit or stuff will get toasted. When looking at the diagrams on thesmartesthouse.com, i dont see it being a option. Same with the zen51.
Actually, I'm saying exactly the same thing. That three plug junction box has two rails in addition to the 3 plugs.
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Lol ok. But that location isn't a concern for the wall wart. It is completely hidden when you put the cover back on. I use a tiny samsung wall wart with my zen 17 down there.
Can the ZEN51 be used in 12-24V or 240V installations?
- The relay part IN and OUT can be used as dry contact, or take 12-24V AC or 120-240V AC. But remember that the ZEN51 device itself needs to be powered with 120V AC and the switch input also requires 120 V to work. If you're looking for a dry contact relay activated by a dry contact input, we recommend looking into the ZEN16 or ZEN17 models instead.
Can I install the ZEN51 without a wall switch?
- The ZEN51 can be wired without a physical switch. You would follow the diagram in the manual, just with the S pigtail (that go to the switch) capped.
- Please ensure you have verified if it's acceptable to have a 120 V fixture without a wall control for emergency shut-off with your local regulations. In most places this kind of set-up is not up to code.
- You will also want to ensure the load is within the specs for the ZEN51: 960W incandescent, 150W LED bulbs, 10A resistive, 1/2 HP fan motors
That right there is the issue. For the fireplace the switch is a low volatge switch. It wont carry 120v that it appears the Zen 51 needs to work with a switch.
The last line in that section is interesting and i suspect is the need. Probably a dry contact relay controlled by a dry contact switch.
The Zen17 has been working for me doing exactly this with my fireplace for a few years now.
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I think the idea would be to power the Zen51 with 120 and wire the dry contacts in series with the existing low voltage wall switch. 
So no wall switch connected to the Zen51 at all.
Edit: Of course it might be handy to have an extra set of contacts for future use. (Zen 16/17)
Of course. Zen58 with wall wart or Zen57 without. Either. I'd personally go with Zen57 if the intent is to use power from that junction. One less item to fail. If you only need one relay the zen17 isn't needed. Won't hurt but isn't needed. That Zen57 is just a newer option.
And when someone turned that switch off?
It would open the circuit. 
Same as it does now.
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