There are very many varieties and sizes Gas Furnaces and can be used for different applications. By and large, the most common gas furnaces use natural gas and utilize electronic ignition. Electronic ignition furnaces are slowly replacing the older style standing pilot furnaces where a pilot light remained lit all the time. Now, with the newer modern electronic ignition, furnaces only use gas when there is an actual call for heat. This adds efficiency because with the older standing pilot gas furnace the pilot remained on even in the summer months. While the heat produced by a standing pilot light is negligible, it is still added heat to the system in the summer when the whole purpose in the summer is to remove the heat from the system rather than add heat like a standing pilot would do.