I’ve been wanting to do this for years but never asked the question. I’m trying to figure out how to change the wall switch out to a thermostat. Does anyone know what I’ll need to do to make this work?
Lots of questions when automatins such a device but
First question. Is there ACTUAL fire in the box or is this a heater that looks like a fireplace? (I see a pilot and assume actual flames)
If so you need to keep the switch but put a thermostat behind it (you can’t / should not have it turn on unattended and at least if you’re in the US, and are in a jurisdiction covered by NEC, remote fireplaces must not be capable of turning on without line of sight verification) (why almost all of the remote operated ones only have IR remotes.)
If it’s an electric heater then you could replace the switch with an appropriate thermo.
So?
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Hey Nathan,
It is an actual gas fireplace. I don’t intend on having it run when I’m not around. I would just like some temperature control when I’m downstairs so I don’t have to get up every time to turn it on and off.
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As @nathancu notes, in most US jurisdictions, adding a thermostat to a gas fireplace/heater requires a licensed technician because of the multiple safety issues involved.
Also note that the safety codes are based on what is possible, not on what your intentions are. If you check this forum, for example, you will see many reports over the years of people saying that lights turned on unexpectedly or even that garage doors opened or doors unlocked when they hadn’t intentionally done anything to cause that. It can happen. 
So in the US, in most jurisdictions, there is a safety code saying that it should not be possible to turn on a gas heater/fireplace if no one is within sight of it, even if you never intend to do that. Unless it is one of those in wall heaters with its own thermostat which is specifically designed to operate on a schedule.
So the first thing is to check with your own town/city to see whether what you are considering is allowed and whether it requires a specific technician certification.
Also, remember that if you intentionally ignore your own jurisdiction’s safety code, your homeowners insurance may not pay off if there is a problem later.
Just something to be aware of. 
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JDRoberts: " there is a safety code saying that it should not be possible to turn on a gas heater/fireplace if no one is within sight of it,"
It may determine the type of gas heater. I have an HVAC system, gas, that I regularly control remotely. Of course, the system itself has all the safety features you refer to.
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As my next sentence says, yes, there are some models specifically designed for thermostat operation that will meet the safety code in most jurisdictions. So as always, the first rule of Home Automation, applies: “the model number matters.”
Some models that are installed without a thermostat can have one retrofitted and meet code, and others cannot.
If I recall correctly, there’s something about a safety valve on the gas line that’s required, but I don’t know all the details. And they may vary from jurisdiction to jurisdiction. I know the requirement about having a certified technician install The thermostat does vary depending on where you live.
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If a device can be classified as a ‘furnace’ it has a different safety classification and CAN earn UL classification operate independently on thermostat because a furnace includes things like a Pilot, Pilot flame out detection and failsafe gas shutoff, Overheat protection and various other failsafe features that ensure ‘off’ of everything isnt EXACTLY right.
Gas log inserts that qualify under NFPA/NEC regulations as ‘furnaces’ can therefore be installed and automated in kind.
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Well regardless they sell a wifi retrofit controller. I was just hoping I could add a thermostat cause it would have been much cheaper.
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Totally feel you on the cost.
I own a gas log fireplace myself, I’m sitting in front of it as I tap this out on my phone… But ex firefighter, and trained as an arson investigator here. No freaking way am I touching the controls on my gas fireplace. I’ve seen way too many well intended ‘upgrades’ that ended in disaster. This is one of those things I absolutely will NOT ever reccommend no matter how easy it sounds.
The most likely modification based on that diagram is just slap a thermo behind the on off switch… But it’s a bad idea because right now. You turn it off and you know it’s off. To turn it on you hit the switch. But you add the thermo - you no longer have a visual indicator that the fireplace is ‘on’ (no flame) if the switch is on and the thermo isn’t at the cutoff temp. Therefore you could walk away from it thinking it’s ‘off’ and two hours later the living room exploded because the pilot failed and a gas leak found the pilot light for the water heater.
BTW you do NOT want to see the aftermath of a contained natural gas explosion.
My long term plan is a retrofit replacement and I will automate that.
This isn’t one of the things to go cheaper on.
Edit because someone asked. Yes this can all happen with a purchased unit the difference is your insurance company has someone to go after for compensation that isn’t you…
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I have a question that I hope isn’t off topic. I am certainly not an expert, but I have had a gas fireplace and an older furnace both with pilot lights that have gone out for significant amounts of time. Doesn’t the thermocouple shut off the gas if the pilot light goes out?
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Absolutely does… (edit if it’s properly designed)
(and yeah I see the question) If you have a pilot driven system intended for operations like that… There’s usually a flame detector sensor just behind where the pilot sits. (it’s one of the but not the only difference that qualifies a gas device as a furnace)
With no flame the primary valve isn’t supposed to be able to deliver gas to the main burner and it should always fail closed.
Basically, No pilot no gas.
In pilot less systems the device needs to detect no main flame and also cuts the gas.
Obviously I’m giving the most extreme situation. But I am not overestimating the fact that the minute your insurance company found out a gas device was ‘modified’ all bets are off. And yes thy absolutely can tell where the flame starts. And if it’s the fireplace the insurance company always demands a second look.
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Thank you for your detailed response.
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Thanks for the feedback and you’re right. My family’s safety is way more important than going cheap on this. I’ll save my pennies for the official kit.
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One of our neighbors did a DIY conversion of a wood-burning fireplace with gas starter to a gas log.
While his installation was fine, it turns out the gas plumbing had a leak inside the wall. I’m told the damage was… um… impressive. He escaped with minor injuries. It took a few weeks for his eyebrows to grow back.
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Ouch.
Ive never actually seen a fireplace ‘explosion’, but I have been on the tail end of an LP tank that was uhm… Used to rapidly cool… Beverages. In a bathtub. While the owner forgot LNG is heavier than air and found the pilot of the water heater. I can’t comment further there was a lawsuit and settlement involved. But… Yeah. *Face-palm. Dude blew himself up quick chilling beer.
Ive also seen a gas dryer become a projectile, literally settling on top of what was left of the homeowners (heirloom, er was heirloom) dining room table (mind you it went THROUGH two ballon-frame drywall over stud walls, punching a Wile. E Coyote style dryer shaped hole as it went) due to a ‘good enough’ DIY job on the gas line install.
Most fireplaces just catch crap on fire - usually a dirty flue - thank goodness… Explosion is rare. Check your well placed (1 in each bedroom, +1 on a common room on each floor) smoke /Co detectors regularly, change the batteries. And replace the device if they’re older than 10 years. 
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For me, the issue is not just the damage to the diy home but the potential spread of fire to surrounding neighbors.
One of the worst fires in California history ($57 million in costs, over 1500 buildings damaged, hundreds of evacuations, 4 neighbor deaths, multiple firefighters injured) was caused by a faulty DIY electrical circuit on a hot tub.
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Thanks! Added to my home maintenance list.
We’ve got a heat pump with electric backup and have always owned electric dryers. But we do have gas stove, gas tankless water heater, gas outdoor grill and gas starters in the fireplaces. All run off a 250gal buried propane tank.
I don’t mess with gas plumbing!
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