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I built one following the directions exactly, and I'm having the same issue, I can only get the temp down to -7C. My water cooling is definitely working as it starts to get warm after a few minutes, but I wonder what you mean by it may not be keeping up. Is there a way to test that it's not keeping up? I've looked at the datasheets for the peltier coolers and they say that the 12710 runs at 15-17 Volts, and the 12709 runs at 15 Volts. With the top peltier running at 5V and 2.5 Amps, the power consumption is only 12.5 Watts, compared to its max of 138 Watts. Could this be the problem that I need to fix, or am I missing somthing? Thanks for your help!!