"The winter of 2013-14 will be remembered by many for the extended range and periods of extremely cold weather. For some people, this is the year the phrase 'polar vortex' entered their vocabulary. While many portions of the world are used to wintertime cold temperatures, there are many that are not, and this may put their fire protection water supplies at risk due to freezing. According to National Fire Protection Association (NFPA) 22, 'Standard for Water Tanks for Private Fire Protection,' where tanks are subject to freezing, methods must be used to maintain the water temperature at or above 42 F (5.6 C) during the coldest weather. (See Coffee Break Training 2012-44.) The tank in the illustration supplies an automatic sprinkler system and is located in a community where the lowest one-day average temperature is between 5 and 10 F (-15 to -12.2 C). Based on the anticipated ambient temperature and tank size, a combination of tank insulation to conserve energy and immersion electric heaters was authorized by the code official in accordance with NFPA 22."
Coffee Break Training - Fire Protection Series No. FP-2014-15
U.S. Fire Administration: http://www.usfa.fema.gov/