So rather than think in terms of how cold a fridge can get stuff, think in terms of how quickly it can remove heat. Air conditioners and commercial water chillers are rated in BTU/hour (in the United States), which is a measure of how quickly they can remove heat. A dorm fridge moves about 100 BTU/hour. A small window-mount air conditioner moves about 5000 BTU/hour (50 times as powerful as a dorm fridge). A 1/6 horsepower commercial water chiller sits in between these figures. It moves about 1500 BTU/hour. Therefore, it's reasonable to conclude that we might succeed in building a reef aquarium chiller from an air conditioner, but not from a dorm fridge. The fundamental heat moving capacity is just too low in the dorm fridge. If you convert a dorm fridge to a water chiller, the fridge will run constantly without making a significant dent in the water temperature, even if you hope to achieve only a few degrees decrease in temperature.

You can get the heat into your dorm fridge in a number of ways, including coiled tubing, an open bucket of aquarium water stored in the fridge, or even by immersing the entire cold heat exchanger in a bucket of circulated aquarium water. It doesn't matter how clever you are about getting heat into the fridge. The problem is that the fridge can't remove it fast enough.

You can do some rough calculations to make my claim more intuitive. Suppose you have a 125 gallon reef aquarium. Neglecting your sump, your 125 gallon tank has about 1000 pounds of water in it. One BTU is the energy required to change the temperature of one pound of water by one degree Farenheit. If a dorm fridge moves 100 BTU/hour, then it would take 10 hours to drop the temperature of your tank by just one degree, and that's if the tank has zero energy input, meaning no lights, no pumps, and perfect insulation from the room. Not very encouraging is it?

Another way to look at it is that one BTU/hour is 0.293 watts. Thus, a dorm fridge that can remove 100 BTU/hour can counteract a heat input of 29.3 watts. That's good for one small powerhead, or a small fraction of a metal halide lamp.

Let's run the same numbers with a 5000 BTU/hour air conditioner as the cooler. That could drop your tank temperature by one degree in 12 minutes, or five degrees in one hour. We're still neglecting the very significant heat input, but these numbers are more promising. 5000 BTU/hour is 1465 watts, so the air conditioner would maintain a constant temperature in a tank (any size tank) that has a heat input of 1465 watts. That's more like what we need for a reef tank.

Building a water chiller from an air conditioner is a tough task, but I have some notes about it here.

Occasionally I hear from someone who claims to have an effective dorm-fridge-based water chiller. I then start asking questions, and I usually discover that, at the same time as the dorm fridge went online, the user made one or more other changes, such as adding whole-room air conditioning, adding a fan to the aquarium blowing at or over the water, decreasing light use, and so on. Occasionally I find someone who believes that he has a working dorm fridge chiller but has made no other changes that would help to maintain a lower temperature. I suspect that these are cases of not having all the information, and that there is always some explanation other than that the dorm fridge really is doing what it cannot do.

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