A wet-pipe sprinkler system is an automatic sprinkler system in which the supply valves are open and the system is charged with water under supply pressure at all times. For this reason, they are the quickest at getting water on the fire and are the simplest to maintain. Wet pipe systems are installed where indoor temperatures can he maintained at or above 40 degrees Fahrenheit. Below that temperature, there is the danger of freezing pipes. If the outside temperature is below freezing and the interior temperature is less than forty degrees Fahrenheit, the steel sprinkler piping, which rapidly conducts heat and rapidly loses it, will drop below freezing. The frozen area may be isolated and near an opening or uninsulated portion of the building. It may be a small area, but it could be enough to put the whole system out of service.