I had the same problem when testing my new set up. I have the 5000 BTU AC with a 48 quart cooler. I had 7.5 gallons of 33/67 glycol solution. When I set it at 20F to crash the test water (1 BBL volume with 50 ft 3/8" ID stainless steel coil inside), it would not go lower than 39F and the chiller...