Electrolux FXTH flexible tank heaters provide low-watt-density heating for viscous materials such as asphalt, fuel oil, pitch and tar, liquid sugar, molasses, lube oils, linseed oil, and other heat-sensitive materials. These flexible heaters are particularly useful for storage tanks that are underground or when the tank ends are inaccessible for installation of more conventional heaters. They can be installed through the normal manhole opening of many large tanks above or below ground without requiring modification to the tank itself.