Yea I agree RMCG, I think it has to do with the total heat transfer necessary. Its not just the total amount of water to be cooled by the chiller, but how much it needs to be chilled. Saying an X HP chiller can cool Y amount of water isn't the whole story. Its how much heat(BTU's worth) needs to be moved. Its just like trying to rate the size of the AC you need for your room based solely on square footage. You can't just say "well this 1 ton a/c can cool a 200 square foot area(or whatever), so I should be good." You need to think about how many KW or BTU's(total heat load) your working with and size the a/c based on that. If I have double the amount of lights as you in the same space, ill need a larger a/c. Yes, the 1 ton might take the room from 90f to 85f, but thats not enough. With the chillers its a little different for various reasons, but pretty similar. The smaller chiller would be running constantly, and the water in the coils wouldn't be cold enough to transfer enough heat out of the UC system. If you know cars, the chiller is just like a radiator. The amount of coolant in the engine is constant, so imagine what would happen if you used a radiator thats way too small. The engine would overheat cuz you can't transer enough heat, but if you triple the surface area of that radiator(or chiller) you'd be good. Also, keep in mind when Ben says he has a 15 gallon reservoir, I'm pretty sure he's talking about the res that's self contained in the chiller itself, not an external one. I thought early on that the chiller systems were limited somehow by the number of turns in the cool coils the growers were using, or in other words the amount of chilled water that is actually in contact with the heat source. But as shown in the large UC grows in the forum, the setup the guys are using work just fine, maybe cuz of the high circulation rates or something. CCH2O knows how to size them correctly. Can't wait for the next update Ben, round 2 looks sweet, that skywalker is a hot ticket item. I was also wondering, why did you decide to double up the air pumps? I know the more DO the better, but doubling up seems like a big increase.