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you would plug it in and leave it plugged in any time its parked....
block heater warms an engine to ease and speed starting and vehicle warmup in cold weather. The most common type is an electric heater that is activated in anticipation of starting the engine from cold, via a standard domestic cord and plug usually routed through the vehicle's grille for connection to an extension cord. The heater itself may replace one of the engine's core plugs, or may be installed in line with one of the radiator or heater hoses. Block heaters that run on the vehicle's own gasoline or diesel fuel supply are also available; these require no external power source. The coolant is heated and circulated, usually by thermosiphon, through the engine and the vehicle's heater core. Heaters are also available for engine oil so that warm oil can quickly circulate throughout the engine during startup. The easier starting results from warmer, less viscous engine oil and less condensation of fuel on cold metal surfaces inside the engine; thus a block heater reduces a vehicle's emission of unburned hydrocarbons and carbon monoxide; also heat is available more quickly for the passenger compartment and glass defogging
Block heaters or coolant heaters are also found on permanently-installed standby generator systems using diesel engines to allow generator sets to handle a workload quickly in an emergency.