Gas Furnaces are special furnaces that consume combustible gases as fuel. As Gas Receptors, they can receive gases from any side other than the top and store it internally as fuel units. The effectiveness of the gas is determined by its Combustibility, where a highly combustible gas will yield more fuel units per unit of gas. Gas Furnaces can store an amount of fuel units limited by their fuel capacity. As long as Gas Furnaces contain fuel, they will burn the fuel regardless of any items being smelted inside or not.
The smelting speed of gas furnaces scales with their temperatures. As a consequence, they are initially slower than ordinary furnaces since their initial temperature is low. The temperature of a furnace can be indicated by the color of the flame inside the grill on the front of the furnace, or the color of the flame in the GUI. The top smelting speed is ten times the speed of an ordinary furnace. Because Gas Furnaces consume the same amount of gas no matter how warm it is, the player can save both time and gas by using a warm furnace.
The amount of temperature units gained per fuel unit burned is the TPF (Temperature Per Fuel). The amount of temperature units lost per tick a furnace is idle is the TFO (Temperature Falloff).
Gas Furnaces are also Gas Propellors, by emitting Smoke at a rate determined by their emission rate. This smoke is propelled upwards, and may be propelled through Gas Pipes. If a Gas Furnace fails to emit smoke, it will stop consuming fuel.
Mods can register special smelting recipes for Gas Furnaces. Glenn's Gases makes it possible to smelt an entire stack of coal into a single diamond, but this process costs a lot of gas as it takes about 4 minutes to complete with a completely warmed furnace. Other features added by Glenn's Gases includes the refinement of Diabaline Ore and the restoration of iron from Rusted Iron Ingots, Rusted Iron Blocks and Rusted Iron Ore.