Shelly Plus Uni self destructed on 24V AC
I recently worked on building an HVAC Temperature Monitor based on this thread: https://www.reddit.com/r/ShellyUSA/comments/1pc61hg/hvac_temperature_monitor/ and the comments therein about using a Plus Uni instead of an Add On. My initial goal was to monitor the lineset coming and going from the indoor coil so that I can monitor for performance drop that might indicate a refrigerant leak before it gets bad enough to freeze the coils. Long term, I also want to monitor performance between the return and supply temps for both heating and cooling.
In any case, I purchased the Plus Uni and a 5-pack of the DS18B20 temperature sensors to get started.
I hooked up a single sensor to the appropriate leads (4, 5, and 7), powered up the Plus Uni using a 9V battery (the myriad threads I found about that have made me question my sanity, by the way), added it to the Shelly App, added the sensor as a module to the Plus Uni, cast the sensor as a device, named it in the app and color taped it to keep track of it, and then added the second sensor the same way.
I've been using Wago 221 connectors, since I have them around and they work with the Plus Uni's minimal-gauge wires just as well as with the sensor wires and power wire I have, although they are comically large compared to the board itself.
Once I had both sensors wired up, named, and working, I transfered the whole kit to a water resistant project box, ran the wires and sensors through stress relief/water-resistant cable glands, and hooked some solid 2/20ga wire to the the power terminals 1 and 2, red and black, also using Wagos.
I found that my power line was too short for me to put the box where I wanted it, so I opened the whole thing up, cut a new, longer line, hooked it back to the Red/Black of the Plus Uni, and then connected the 2/20 to the R and C terminals on my furnace's board (the same terminals that the thermostat is hooked to).
I re-energized the furnace and immediately smelled burning and saw smoke coming from the Plus Uni's circuit board. I let go of the safety switch on the furnace to cut power and immediately unhooked the 2/20 wire and pulled the whole thing out.
I have read other threads indicating that this power scheme should be safe and sufficient and others that have seen people fry their boards.
Is there any guidance or assistance that can be offered on figuring out if this is supposed to be able to work this way or if I possibly got a bum unit? Or maybe the burned component is an indication that there was an unintended short in the box somewhere?