Minecraft wiki entry for Automatic_smelting, associated text: Variations: You may choose to add switches to control the hoppers. Switching off the bottom hopper stops it from taking items out, allowing you to collect the experience from smelting them. Switching off the top hopper allows you to fill the top chest with various items, then eventually smelt them all in one go, reducing the fuel waste normally incurred by smelting partial stacks. The fuel hopper on the side never needs to be switched off.