Furnaces are a heating device and is an upgrade of the heating stand. While they can take burning wood as fuel (heating stands can too), they can also use coal, which is 8 times more efficient. Additionally, their closed nature allows one to smelt more toxic materials, but one still needs a container for which to hold the toxic material afterwards.
6 (40 vs wall)
Electric furnace (furnace + thermocoil + electric adapter)