Hmm, didn’t think about all of the bad things mentioned above when I did mines. 
Since my fireplace has a switch to control the gas valve on/off (no 120v) and a switch to control the fan (120v), I wired a 120v to 24v transformer from the outlet (powered by the fan switch) and a 24v coil relay from that to open and close the gas valve circuit (basically another switch in parallel with existing switch). I connected a smart switch to the fan (120v) which allows me to turn on/off the fan (smartly).
So when the fan is supplied power, the transformer sends 24v to the 24v coil relay which closes the circuit which opens the gas valve. When the switch is off the relay opens which turns off the gas valve switch… This is a 100% mechanical process. The actual fan, which is clueless of the “extra stuff”, doesn’t come on until the fireplace reaches 120 degrees via a thermal coupler that’s mounted to the underbelly of fireplace.
Con, outside of the many listed above:
The existing wall gas (on/off) switch still works but if the smart switch turns on the fireplace then it can not be turned off via the old switch.
I have 2 temperature sensor and 2 motion monitors in the area of the fireplace and I have 5 CoRE safety pistons.
- If fireplace stays on for 3 hours, turn off
- If smoke detector detects smoke, turn off
- If temperature reaches 74 degrees, turn off
- If presence sensors (3) are not present, turn off (this makes it so you can’t turn it on remotely if no adult is home)
- If motion sensors stay inactive for 30 minutes, turn off
I didn’t worry about the fan speed (set with variable motor speed controller inside fireplace) nor the intensity of fire (set by the gas valve inside fireplace).
Todo (Maybe):
Find a 24v coil relay that work in conjunction with a 3-way switch. So either the existing (dumb) or the smart switch can turn the fireplace on/off no matter what.
Works pretty darn good but I do understand the safety and code issues this poses. Proceed with great caution!
