I asked a similar question of Paul Hall, Affordable Distilling, his answer is below. Some background first though. We get our water from Lake Erie and it is reasonably expensive. The temperature of the lake in August is 75 degrees.
Most of my customers use tap water to chill their fermenters. Exactly how much chilling your fermenters need depends on a number of different variables including, but not limited to the temp of the air in the room, what you are mashing. Even if you are mashing the same grain bill the mash will build up different amounts of heat depending on which yeast that you use. Also it depends on what max temp that you are holding your mash to. Some people don’t let their fermentations get over 80 F while others let them get up to 95F. So there is absolutely no way that I or anyone else will be able to tell you exactly how many btu’s that you are going to use. If you can pump and use lake water that is free or if your tap water is really cheap then I wouldn’t worry about a chiller as long as the water is 72F or lower. If your water is expensive then and over 60f then I might use a chiller.
If you think that you will have air temps in the distilling area over 95 f in the summer then I would get a 1 ton chiller. Otherwise a ½ ton will do the job no problem at all. If your tap water is really cheap you will spend a lot more running a chiller than using 70F tap water.
Please keep in mind that your fermenters will take very little cooling energy compared to your still condensers and crash cooling corn or rye mashes when using the cook method. If your tap water is cheap you can use 68F or colder tap water for you condenser cooling and it will come out of your condensers hot. If you are smart you will run that free hot condenser water into 2 hot water holding tanks. Use the water from 1 tank for your next mash cook and the hot water in the other tank for cleaning around your distillery. The cooling specs for the 300 gallon stripping still are 55° F water in 150° F out at 3.4 gpm Max. For the 100 gallon still the input condenser water at 55° F and 150 ° F out with a flow of 1.2 gpm max. Many people in the industry use close loop chillers. I would never do that. I think that it is crazy to return 150 F water back to the chiller to cool it down to 55F. That is a huge waste of free hot water and energy. If you just have to have a chiller, I would chill a tank full of water, run it through my condensers and then use the free hot condenser water that comes out of the condensers