Heaters are for when the water temp drops too low...has nothing to do with the chiller. The chiller is to keep your water at that same temp, when it trys to climb higher. This could be due to lights, summer weather, etc. The heater is self explanatory...it kicks in and gets hot, warming up the moving water around it when it senses that the temperature has dropped below the setting that you desire. A chiller is much like a refridgerator, in that it senses when the temp rises above your desired setting, thus turning on. But most chillers have "lines" that pass the tank water thru them, and "cools" the water as it passes, then returning the water to your tank.