The Cool Control is a thermostat that switches the power on when a set temperature is reached and off once the temperature has fallen below the set level. It is like an ordinary thermostat but only in reverse. It is commonly used to switch on cooling devices and thus prevent overheating. Fans like the HabiStat MiniFan are typical of the devices controlled. Now with both device on and power indicators. The power indicator will shine whenever power is applied to the unit and the device on indicator will shine whenever power is being supplied to the fan. Namely, when the fan is on. The cool control is an ideal safeguard against heat build up on warm days. It is particularly useful when used to evacuate hot air out of marine fish tank hoods. This unit may not work on fans with variable speed controls and the manufacturers of the fans should be consulted.