Technically, an unheated fountain will freeze at 32 degrees Fahrenheit. The amount of time that it will take to freeze depends on how well insulated the unit is, how much water is in it, and the beginning temperature of the water. Testing of a Thrifty King CT unit showed that at 20 degrees below zero Fahrenheit with a 15 mile per hour wind, the water temperature inside the unit dropped ½ degree per hour. We recommend a minimum incoming water temperature of 42 degrees Fahrenheit for Thrifty King units. Under the conditions mentioned above (42 degree source), it would take 20 hours of non-use before the water inside would freeze. Open trough fountains will freeze much faster and must be heated in freezing environments.