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). Unlike the Bread Oven, there is no [verify] limit to the amount of wood you can deposit.
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 a full night of heat, 48 wood will be burned by the Furnace. Try not to dump in all of your wood when not going AFK, or it will get wasted during daytime. When going AFK, make sure that you are within the warmth range of your furnace, you have auto-feed on, you are atop a bridge with a body of water under you, you have strong walls/spikes defending your position, and all surfaces inside your defenses are covered in bridges and roofs to prevent mob spawning and attacks from flying mobs and players. You will need 12 wood every minute you go AFK, or 720 wood every hour.