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A typical block heater cord.

A block heater or headbolt heater is an electric heater that heats the engine of a car to ease starting in cold weather. They are connected to normal AC power overnight or before driving, via regular power plugs that are fed through a vehicle's grille. This allows easier starting in cold temperatures, by keeping the coolant warm. Heaters are also available for motor oil so that the oil does not gel and its viscosity remains low. The rapid heat circulation caused by block heaters also aids the initial vaporization of fuel in the engine; because of this effect, block heaters reduce pollution, as poor fuel vaporization causes much higher emissions.

Block heaters are widely used in many northern U.S. states, and are very common in countries with colder climates like Canada and Scandinavia. In colder climates block heaters are often standard equipment in new vehicles. In extremely cold climates, electrical outlets are sometimes found in public or private parking lots, especially in multi-storey car parks. Some parking lots cycle the power on for 20 minutes and off for 20 minutes (e.g. the Light Rail Transit lots for a number of Canadian cities), in order to reduce electricity costs. This results in a decrease in heating effectiveness for the average block heater. However, to what extent this reduces block temperatures in −30 degrees Celsius (−20 degrees Fahrenheit) weather is in question.

Block heaters are usually inserted into a freeze plug manufacturing hole in the cylinder block, heating the metal, coolant and oil. These can be installed at the factory, or by any qualified automotive service center. A block heater can even be installed by the handy do-it-yourself'er.

There are many alternatives to a block heater that have some of the same benefits. A particularly simple option is to buy an after market heater that is attached to the oil pan, sometimes with magnets. Also, one can even get a heated dipstick. Another alternative to block heaters is a battery warmer, which keeps just the battery warm. This maintains the efficiency of the battery, which is highly dependent on temperature, and is cheaper than heating the entire engine block. Heated blankets are available for the entire engine area, as well. A standard timer switch can be used with any of these heaters, so that it does not have to be left on all the time. This can help lower the electrical costs of owning and using a block heater.

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Andrew Freeman, a North Dakotan, invented the headbolt heater in 1947 and received a patent for it on November 8, 1949.[1] Before the block heater was invented, people utilized various means of warming engines before starting them, such as pouring hot water on the engine block or draining the engine's oil for storage inside overnight. Some even shoveled hot coals underneath their vehicle's engine to achieve the same effect.