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Not sure this is a "home improvement" question, I want to build a refrigeration room and instead of buying some industrial cooling aggregate through complicated distributor sales channels I am thinking of taking a normal AC split unit, and modify the thermostat. I wonder if this is common, if there are some brands that allow you to hack the thermostat, or if I should just be able to take the PCB and reverse engineer it just enough to identify the thermostat and ultimately just the one-off output of the thermostat so I can hook up my own (in a better position of the room). Doable? Do the manufacturers still include schematics as they used to 50 years ago?

To address comments: obviously the insulation is paramount. And I am in a perpetual warm zone, so there is never frost outside. Also it's a rural place and not even in the USA so whatever "code" doesn't matter.

EDIT: there is nothing opinion based about this question. It asks if it can be done. Received a balance of 3 up-votes and 2 answers. One answer said that messing with the thermostat is quite easy, but to achieve cooling below 10°C one may have to make additional adjustments that are not just in the control electronics.

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    Before worrying about thermostats, you need to be able to insulate the room enough. Today you are lucky to get a contact manufacturer for more than plug in and hope it works. There might be hacks(unsafe) to add/use a fridge/freezer thermostat, that will go down enough but will be unsafe/not code/illegal/not recommended/end up as a darwin award.
    – crip659
    Commented Sep 4 at 21:09
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    I agree with @crip659 that insulation is important. In our case, we used conventional stick framing with 3+ inches of spray foam insulation. We used an insulated exterior door for the cool room entrance. This seems to work well, although on humid days we get lots of condensation on the metal door knob and even on the concrete slab around the outside of the cool room.
    – Rob
    Commented Sep 4 at 21:22
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    For what it is worth, I agree that this is not opinion-based. I don't have enough reputation here to do anything about it, though.
    – Rob
    Commented Sep 9 at 17:59
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    This is not enough for an answer, probably, so: why do you not salvage the refrigerating parts of a freezer? It will surely achieve refrigerating temperatures, even though slower...
    – virolino
    Commented Sep 10 at 11:22
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    It's a fine question and this isn't an answer but you can buy a new restaurant reefer head for about $4,000. As for complicated sales channels, worst case you get a free lunch and a hat so what? But you can buy them online. The mini split won't work, won't cost that much less, and whatever goes wrong with it will be your fault whereas the proper one has a 2-year warranty.
    – jay613
    Commented Sep 10 at 12:43

2 Answers 2

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There is a product I use to cool my brewery's cooler using a mini split (although it works with other types of air conditioners as well), called a CoolBot. The basic idea is that it tricks the AC's sensors into cooling much lower than it normally would, and has sensors of its own that allow it to manage defrost cycles.

The CoolBot comes with detailed instructions, so this can be a DIY project (although I had my HVAC guys that were installing the mini split system do the CoolBot install as well).

In combination with a "commercial" mini split that will cool even when it is very cold outside, my cooler stays at 35F year round.

I am not associated with the company, aside from being a satisfied customer. I expect that there are other similar products out there. Or you could take the idea and run with it an do a fully-DIY version for yourself.

A couple of related issues that I discovered along the way:

  1. I had to move the condensate drain from outside the building to an inside drain, because as soon as winter hit and we had freezing temperatures outside, the drain froze and overflowed from the mini split head.
  2. I have to run a dehumidifier inside the cool room. Make sure to choose the right dehumidifier technology that can work effectively at these low temperatures.
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  • Why a dehumidifier? The cooler should dehumidify. And the dehumidifier was just an air conditioner with the outside part integral to it. It heats the room. Seems silly I'm interested why.
    – jay613
    Commented Sep 10 at 12:45
  • @jay613 I was surprised too. I don't understand the mechanism, but empirically it gets and stays super humid in there without the dehumidifier. Like >85% humidity (at 35F). I remove almost a gallon of water every 24 hours during the summer months. I know humidity gets in with me opening the door to get kegs, etc. My guess is that all condensation produced during chilling immediately freezes and is only captured on the defrost cycle. So, it can't dehumidify continuously like a normal mini split head would do during cooling?? I would be interested in other people's thoughts...
    – Rob
    Commented Sep 11 at 14:35
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    This is a DIY forum. All the conventional wisdom about problem solving with air conditioners and dehumidifiers is based on operating and typical living temperatures. EG typically an oversized A/C creates dank (cold, humid) air because it doesn't circulate the air enough to remove moisture. But at 35 degrees a mini split should not be expected to perform normally, and "conventional wisdom" won't always work. This question might be better on the engineering stack which has refrigeration tags.
    – jay613
    Commented Sep 11 at 16:09
  • @jay613 I agree. I don't understand why I have a humidity problem, but I can measure it and fix it. I had to research the right dehumidification technology that can work in a 35F space (conventional dehumidifiers are ineffective), and it does indeed work.
    – Rob
    Commented Sep 11 at 16:19
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A normal split air conditioner unit is very simple to control: just short the Y terminal to the R terminal. Maybe G to R also, depending on whether the air handler/furnace turns on the fan for cooling automatically or not. So modifying the thermostat is easy -- you can set the thermostat aside and use a jumper wire or wire nut to start experimenting right away.

There's more to it than just modifying the thermostat though. A normal home split system is balanced (meaning air flow through the evaporator, refrigerant system cooling capacity, refrigerant charge/pressure, and TXV operation or orifice size) so that the evaporator coil runs somewhere around 48 °F. If you were to wire it so that the A/C would run indefinitely, and if the insulation is really good and the heat gains are small, I think you might cool the house or room or whatever to somewhere around 50 °F.

To go lower than that you'll probably need to modify the TXV or install a smaller orifice, or reduce the refrigerant charge in the system, to reduce the evaporator temperature. As for whether a conventional packaged compressor-condenser outdoor unit will suffer any adverse effect for running its suction side at lower-than-normal pressure, I don't know!

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