Building your own chiller is a very challenging project, and after trying it a few times, I found the $300 average price of a used chiller very inviting. That's what I ended up doing; I bought a used chiller.

The basic idea for building your own is to build (from acrylic, say) a watertight box around the evaporator coils of the air conditioner, and then pump aquarium water through that box. The evaporator coils are the part of the air conditioner that gets cold. They have aluminum heat sink fins attached, so the whole thing looks like a radiator. You want to tear off the aluminum fins, which you can do fairly easily with pliers. Then clean the bare copper pipes well. I recommend some combination of sanding with fine sandpaper and using a degreaser. Chemical etching might be even better than sanding, but I don't think that's required. I didn't do it. Then give the copper pipes several coats with an epoxy paint or epoxy resin.

My final DIY chiller attempt, pictured above, was real close to being adequate. I went wrong in two places. First, the epoxy resin I chose was too brittle, so it developed small cracks over time. I don't know if the cracks went through to the copper, but I didn't like it. I used the 3:1 ratio epoxy from Fiberglass Coatings Inc.. If I did it again, I'd use their 1:1 ratio epoxy, which is more flexible, and in fact can be mixed deliberately off ratio to control the flexibility. I might also wrap the copper pipes in fiberglass cloth as a part of the epoxy coating stage. I'm talking about making a fiberglass-epoxy composite coating, which would have much more strength than epoxy alone. Fiberglass Coatings sells fiberglass cloth and tape (thin strips). Don't make your coating thicker than necessary though, because epoxy (and glass) are not good thermal conductors compared to metals. I would try using a single layer of cloth no heavier than 4 ounces per square yard (that's how fiberglass cloth is rated - in ounces per square yard).

My second mistake was trying to preserve the sheet metal end pieces of the heat exchanger. The copper tubes pass through these end pieces, and the end pieces serve to support the tubing. If I did this again, I would carefully cut away the end supports, and build some sort of support out of acrylic. Then my acrylic box would completely encapsulate the copper tubes and my new supports.

The metal end pieces made it difficult to get a good seal of epoxy between the sheet metal and the copper. After a few months of chiller operation, I noticed little bits of corrosion. Nothing bad happened in the reef, but I didn't like the corrosion, so I just stopped using the chiller.

During its short service time, my DIY chiller was more effective than the commercial reef chiller that I use now. My DIY chiller started life as a 7800 BTU/hour air conditioner. A rating of 7800 BTU/hour is about what you get from a 3/4 HP reef chiller. But the 7800 BTU/hour rating of my air conditioner indicates how quickly it cooled air. It may cool water faster because of water's greater thermal conductivity, or it may cool the water slower because the epoxy coating is an insulator. I didn't do tests, so I don't know which way it is.

I used a commercial (Goldline brand) digital temperature controller to control the reef tank temperature. It turns the air conditioner on as needed to maintain the setpoint.

It's tempting to make another attempt at a DIY chiller, but other projects are more compelling right now.

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