Furnace Heaters are a block from the Factorization mod that let Slag Furnaces cook without coal. Instead, they use electrical charge from a Solar Turbine or Battery Block. A furnace heater can be connected to Factorization power using a Lead Wire or by simply placing it next to a powered machine.
Furnace heaters will also power vanilla furnaces. Adding multiple furnace heaters to a single furnace or slag furnace will considerably boost its smelting speed.
They are also used with the Crystallizer.