The furnace gives the player warmth, like the Campfire and Big Fire. Unlike the Campfire and Big Fire, the Furnace does not disappear after a certain time; players can add wood to it to restart the fire. Also, standing near the center does not harm the player (unlike the Campfire and Big Fire).
The furnace behaves like a one-way-chest. Players can put any amount of wood into its inventory, but it is not possible to take it out again. As long as the Furnace contains at least 1 wood, it will give out heat. However, 1 wood will "burn" every 5 seconds, and disappears from the furnace's inventory. When empty, it only acts as a wall (albeit a wall with less health), being impassable.
For each night, around 40 wood gets used as fuel in the Furnace. Try not to dump in all of the wood, or it will most likely get wasted during daytime. When going AFK, make sure that you are in the warmth range of your furnace.