It’s a small volume, and it’s insulated? Whatever the purpose is, a window unit AC would be overkill. I would expect short-cycling would be occurring. (OTOH, I’m not sure what a good alternative would be. Peltier module, perhaps?)
Yeah, I’m a little doubtful of the fermentation idea based on the size. With added insulation inside the box, I can’t see how you can fit a fermentation vessel in it. Certainly the typical 5-gallon carboy doesn’t fit.
Mid-60s temperature is fine for ale or wine fermentation. Like you say, it’s too high for lagers. I can see someone making a 65-degree box if they keep their house warm like the thread Bear_Nanno linked, but then why not build one you can actually fit something in.
New guess: maybe a small-scale wine cellar?
@ShadowFacts, can I ask what your climate is like? How hot does it get in summer, how cold in winter?
I’m in the Northeast, so it gets damn cold in the winter and damn hot in the summer 
To be more specific, summers generally peak in the mid-to-high 80s with several heatwaves in the 90s plus humidity. Winters in the 30s/low 40s with snow and some stretches of below freezing.
You keep saying that word. I do not think it means what you think it means.
ISWYDT
Boy, that’s for sure. It will only run for about 30 seconds.
But even at that, keeping your house around 70 is a whole lot more doable than rigging up a contraption like that just to keep it marginally cooler.
This is what we use for a walk-in cooler to store produce prior to sale:
The air conditioner could just be put on constant operation so that it never shuts off. Depending upon the leakage of the container and the effectiveness of the insulation, it could easily keep the interior in the low fifties (Fahrenheit) or even lower; not sufficient for refrigeration of meat or dairy products but just fine for bottom fermenting beer or white wines. This wouldn’t be very efficient, and if I were designing a box for this purpose I would definitely use a Peltier thermocooler with a couple of wireless IC sensors inside the box or mounted on the items being cooled, but this entire setup looks pretty janky so probably assembled by someone without a lot of expertise.
A 5 gallon carboy is about 20 inches high and less than 12 inches in diameter. Even if the entire interior surface of the box were covered with 2 inch foam board insulation there should still be plenty of room for three carboys with plenty of room to spare for circulation.
Stranger
I just measured my carboy. With a stopper and airlock on the top, it’s a little over 24 inches. The interior of that storage model is 25 inches, so I still say it’s not appropriate for typical home fermentation.
I can’t find explicit manufacturing specifications for food-grade carboys (my Fisher and other glassware catalogs only list chemistry glassware in liters or milliliters) but every supplier I can find for glass beer brewing carboys lists 19 to 19.5 in height and 10-11 inch in diameter for a 5 gallon carboy. A few examples below:
Stranger
Yes, I have a few and mine are all ~20 inches. But when fermenting, you need a stopper and airlock (bubbler) on top, and those add another ~4".
So you cut clearances in the insulation, or just don’t insulate the top and bottom. Or maybe they’re using 3 gallon carboys and splitting the mash between them.
Stranger
Yeah, all possible. It’s not that it won’t work for fermenting. But if you’re building something specifically for fermenting, why not start with a container that works for the most common scenario?
This looks like they started with what they had on hand or could salvage at low cost. Whatever it is intended for, it’s pretty slapped together.
Stranger
THE EXCITING CONCLUSION!
Before I mentioned to my wife the speculation that it was some kind of home brewing operation, she walked by it earlier today and said it “smelled like someone spilled a beer.” So we just undertook a more rigorous reconnaissance mission together. We slowly walked by in both directions and breathed deeply and … yes! There is a distinct hoppy scent in the area, subtle but noticeable, particularly on the side away from the A/C unit.
So I am calling that conclusive enough for me: it is a home brewing operation of some kind.
Many thanks to everyone who made suggestions - well done! Not knowing was driving me a little crazy
You know, this wasn’t the mystery-orb-behind-the-building of many years ago, but it was pretty interesting.
As a former beer brewer and charcuterie maker guy, the strangest thing about this to me is that refrigerators are just dirt cheap to free on Facebook marketplace. I’m sure it cost more to build this contraption than to just haul home an ugly fridge.
Yeah, it doesn’t make a lot of sense, nor does its placement in the front of the house.
A window A/C does not operate like that. All you can do is turn the set temp down as low as it goes. When it gets that cool in this little box it is going to run for a few seconds. My mini split has a dry mode that is different from a window unit. It just runs on a fixed cycle like 8 minutes on, 3 minutes off. It is is the setting I use almost all the time on mine. As far as I can tell it has no minimum shut off temp as I have found out a time or two.
My house is in the woods and always has high humidity outside compared to more open areas. I have studied and modified window units before I got my mini split. I have perfected a simple change that provides perfect temperature control and minimum humidity. NO MANUFACTURER OFFERS SUCH A FEATURE.