At subzero temperatures, the frost protector prevents ice from forming in the filtration cycle.
For use of the filtration system at subzero temperatures, the frost protector and the insulated lid are required as frost protection. Ice can quickly form in the filtration unit, which is why the frost protector must remain activated at all times.
The water flow created by the pump is what activates the frost protector. Simply use the thermostat to set a target temperature. The frost protector will activate when the temperature goes below the target. It will then shut off and repeat the process.