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I live in a tiny house of sorts and my AC doesn't run while I'm not there. I have two desktop servers that I use for my work which are currently stored elsewhere, but I'd really like to move them to my home. My only problem is that I'd need to supply the computers with cool, dry air (think 60-70 degrees) while the rest of the house gets over 100 degrees F during the summer months.

I've designed a basic insulated box (R19) with a vapor barrier on the inside to fit my computers. It's a basic box with some mineral wool insulation and some plastic sheeting between the insulation and the computers.

I have the following already:

I need help with the following:

  • What small, efficient appliance can I use to cool and dry the air going into the box? basically a mini AC + dehumidifier
  • Is the vapor barrier in the proper place to keep water condensation from damaging the computers?

I've tagged this under different appliances, but feel free to change if there is a better tag for it.

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    Vapor barrier typically goes on the warm side. You don't want moisture passing through the insulation and condensing on the outer surface of the plastic, which won't affect the hardware within but will grow mold. The problem with cooling appliances is that they need somewhere to dump the heat. Your refrigerator dumps it into your kitchen. Your air conditioner dumps it outside. What's your plan?
    – isherwood
    Jul 2, 2019 at 17:36
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    You've answered the question yourself. All air conditioners dehumidify as a natural result of cooling, and you'll just need to find a unit that fits your situation. Specific product recommendations are off-topic here.
    – isherwood
    Jul 2, 2019 at 17:45
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    If this is a tiny house, consider just cooling the whole thing down to 80 degrees with an A/C. An A/C does humidify, but it has to run a certain amount of time to make a meaningful difference in the humidity. Trying to cool a small box with a room-sized A/C would make it "short cycle" which is inefficient and won't help the humidity.
    – JPhi1618
    Jul 2, 2019 at 18:22
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    For all that trouble, you might as well just water-cool them.
    – Harper - Reinstate Monica
    Jul 2, 2019 at 20:47
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    What's outside air temperature like? A filter and pair of fans to outside (one intake, one extract) running continuously might be a cheaper/easier option.
    – SomeoneSomewhereSupportsMonica
    Jul 3, 2019 at 3:10

3 Answers 3

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You need an ultimate-heat-sink OUTSIDE the house

Here's what you can't do: Server makes heat in box, A/C moves the heat out of the box and into the tiny-house, tiny house copes with ambient + solar load + server load + A/C load all stacking to make it so hot you can bake cookies. At that point, the A/C, will break down due to not being able to transfer the heat from the Freon to the too-hot environment.

So every solution must remove all server and A/C heat entirely out of the building -- and if it removes solar load as well to make the place habitable, that is fine.

Put the box on an outside wall

In this case, you mount the air conditioner on an outside wall, and enclose the A/C cool side with the servers. This will have a big problem: accessing the A/C controls. A wired remote will cure that; a remote thermostat won't since the thermostat will be in the wrong place. A wireless thermostat may have trouble with UV line-of-sight. In this case the A/C unit needs 3.4 BTUs per watt consumed by the servers, just to break even, and that assumes ideal conditions. So you would be wise to upsize the A/C unit somewhat.

A carefully oversized unit would allow you to open the house-side door to the server box, and let the unit cool the entire house.

Downside: It will be L-O-U-D all the time. It will be like sleeping with a running vacuum cleaner. Your neighbors won't like it either.

Big downside: When the A/C cycles off, you have a problem. It must wait awhile before restarting, due to a problem called "short cycling". Meanwhile the box will very quickly heat up, and as it heats up, cooling fans/heatsinks becomes even less efficient, creating a vicious cycle. The box could critically overheat before the A/C is ready to run again. The way you solve this is by having a much larger "thermal mass" to share the heat with. But water jugs won't cut it because they have too little surface area to interchange heat quickly. You need something with a lot of surface area, like... the rest of the house.

Air condition the whole tiny-house 24x7

In this case you don't bother with a box, just mount the servers where convenient and mount the air conditioner(s) where convenient. As far as air conditioner size, you will need the normal BTUs + 3.4 BTUs per watt of power actually consumed by the servers. So if you have 3000 watts of servers you'll need 10,200 BTUs additional, on top of the normal BTUs your tiny house will require. That math works because servers convert 100% of their power to heat.

Downside: you are paying to cool the entire house all the time, even when you are not there.

Water cooling on the servers

In this case, you fit water heatsinks onto the major heat-making components, and plumb them so you can carry the hot water outside.

Skip through what these guys did. They did a lot of mistakes and wheel-spinning that totally defeated the effectiveness of the project, because they are dumb and it was a stunt for a video. But the principle is good.

I'm not a fan of how they use copper instead of cheaper stuff like PVC (humans won't drink it; no need for PEX). In fact, the long uninsulated copper pipe runs acted as a radiator and dumped all the heat right back into the room, completely defeating their project. They also failed to design "high spots" for bleeding bubbles. All easily fixed. Also, keep in mind these guys promote products for a living, so for instance they did a very stupid radiator design simply to use that manufacturer's units, which were totally unfit for the application.

A sensible person would go to a pick-your-part type junkyard, and obtain a radiator assembly off an SUV. Including radiator, shrouds, fans, thermal switches, fan relay(s) and wiring harness bits. Wire it so when the thermoswitch closes, it starts up the fan(s), which you power from an adequate 12V power supply. All this stuff is designed to live outside, obviously.

An SUV radiator is so large that I doubt the fans would ever need to run if it's not in direct sun. That means the cooling system runs near silent. The only thing running is a water pump.

Also, you would never use plain water, not least because stuff grows in it. Use propylene glycol antifreeze (won't kill cats and dogs), or mineral oil.

Downside: This only cools components with cooling plates on them. The rest of the components will still be adding heat to the box, and that too must be removed. You could help that with another radiator inside the box, and putting it "first in line" for cooling: so the cooling flow is external radiator -- in-box radiator -- CPUs -- VPUs -- external radiator. Water cooling is so darned efficient that it won't matter if the water arrives at the CPU somewhat hotter.

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  • LOOOOL I knew that video was whole room water cooling before I even clicked on it. I was cringing in wait while my slow internet loaded!!! Practically though I would have to water cool a dozen hard drives....less than ideal. I'm leaning toward AC-ing the whole place.
    – Joe B
    Jul 3, 2019 at 5:41
  • I didn't realize anyone put heatsinks on hard drives. I was thinking CPU/GPU...
    – Harper - Reinstate Monica
    Jul 3, 2019 at 6:22
  • Also you really have to watch part 7 @JoeB where he lists all the mistakes he made, e.g. selecting copper and not insulating the long run of pipes, so the heat doesn't really leave the room after all. The guy was his own worst enemy, honestly, but we don't need to repeat his mistakes.
    – Harper - Reinstate Monica
    Jul 3, 2019 at 15:03
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How much cooling do you need?

The biggest factor in sizing a unit is the amount of heat that needs to be removed, measured in BTUs. A server (excluding "Raspberry Pi as a tiny server" since obviously that's not what we're talking about here) might use anywhere from 200W to 1,000W (possibly more). If you have 300W x 2 x 3.4 BTU/kWh = 2,040 BTU/hour. With a typical small 5,000 BTU window unit, the air conditioner would cycle ~ 40% on, which is not too bad. Of course, heat from the rest of the house - i.e., solar energy - will need to be factored in as well. But unless you have really big servers, stick them in a room with a window, install a window air conditioner and you're all done for a lot less than a real "custom" solution.

Window Air Conditioner

There are plenty of specialty units out there. But the cheapest functional solution is likely to be a small window air conditioner. Since this needs to vent heat to the outside, the simplest installation would be to put the servers in one room with a window, install the air conditioner in the window and keep the door closed and the air conditioner on when you're not home. No need for special thermal controllers or boxes or anything else - the room is the "box" and the thermostat in the air conditioner takes care of the rest.

The next best solution is to use a window air conditioner and set it up so the cold air output goes into the "server box". That complicates things quite a bit as (a) even a small window unit is may be more cooling than you need and (b) you need to make sure that the thermostat is within the conditioned space as otherwise it will not cycle properly.

Portable Air Conditioner

The last solution is a portable air conditioner designed to pump the hot exhaust air out through a flexible duct. The catch is that a unit like this is generally larger and therefore more expensive to purchase and will likely be too large for your application, resulting in short cycling which is not good for any air conditioner.

In a very quick search, I easily found small window units for < $200. I think that is the best solution here.

Dehumidify? That's included.

As far as dehumidify, as others have commented, an air conditioner is, by its nature, also a dehumidifier. Cold air holds less water vapor than warm air. An air conditioner cools air so much that, unless you start with very dry air, water vapor condenses out of the air. This condensate, if not properly drained, will make quite a mess - which is the subject of many other questions. But functioning properly, any air conditioner will dehumidify quite well.

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  • The air conditioner needs to remove 3.4 BTUs per watt of server power draw, on a continuous basis.
    – Harper - Reinstate Monica
    Jul 2, 2019 at 21:10
  • @Harper True, but the actual power usage is often (unless you're mining) far less than rated power supply. Could be anywhere from 200W to 1000W per server. If it is 300W x 2 = 600W x 3.4 = 2,040 BTU - so a small (< $200) 5,000 BTU window unit running at 40% on would do the trick. I'll add to the answer.
    – manassehkatz-Moving 2 Codidact
    Jul 2, 2019 at 21:24
  • I'm not at all sure the % run factor is going to work. The instant the A/C cuts off, the temperature in the small box will climb fast. Meanwhile the A/C unit is sitting there running down its anti-short-cycle timer. By the time it restarts, the box will be quite warm.
    – Harper - Reinstate Monica
    Jul 2, 2019 at 21:37
  • @Harper, the short-cycling problem could be solved by adding thermal mass inside the box, such as a (well-sealed) bucket of water.
    – Nate S.
    Jul 2, 2019 at 21:44
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    I wouldn't run it in a small box. I would run it in a small room.
    – manassehkatz-Moving 2 Codidact
    Jul 2, 2019 at 21:51
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The only thing inside a computer that shouldn't be greater than 37 degrees Celsius (100 degrees F) is a spinning hard drive.

Instead of spending money on a cooling solution, spend the money on SSDs to replace the hard drives. Do that and you're good to go.

50 degrees C (122 degrees F) is the temperature I'd shoot to stay under.

A few commenters mentioned water cooling, but typically that only cools your CPU and maybe your VRMs if you get an expensive water block. These two things are actually the two components that don't care much about running hot. Typical CPUs can handle over 65C, and VRMs are happy at over 90-100C.

Like I said it's just the moving parts like the fans and hard drives that don't like heat, but fans are cheap and hard drives can be replaced with SSDs for less than the cost of an AC unit.

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  • That's wrong. CPU care a great deal about running hot. Heat removal is the limiting factor in performance and has been for 20 years. They care so much they have internal temperature sensors and will reduce performance to avoid thermal self-destruction. That'll fool you; the system will boot and "run" with the heatsink completely removed, but the performance will be totally unacceptable.
    – Harper - Reinstate Monica
    Jul 3, 2019 at 14:54
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    @Harper, that's true, but throttling doesn't really kick in until much hotter -- throttling doesn't even really start until you're above 50C, and isn't significant even up to 90C. So in theory, with a powerful enough air cooler, running in 37C ambient would be fine. The real problem is that in a small sealed room, ambient won't stay at 37C as the computer dumps heat into it. Source: pugetsystems.com/labs/articles/… (& also I'm an electrical engineer at Intel)
    – Nate S.
    Jul 3, 2019 at 16:01
  • @NateStrickland The problem is, it's a long way (thermally) from the heatsink face to ambient air: you need a big thermal differential to get the heat to move. When that thermal differential gets smaller, heat flow gets smaller too, i.e. heat sinks lose efficiency. Inside a box, that makes a vicious cycle.
    – Harper - Reinstate Monica
    Jul 3, 2019 at 16:38
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    @Harper, for sure, the "with a powerful enough air cooler" was the key phrase there. You can get away with a low temperature difference if you have a whole lot of airflow, but this isn't a very popular choice (outside of server farms) because it's LOUD.
    – Nate S.
    Jul 3, 2019 at 16:45

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