Furnace

The Furnace is a Game Mechanic in some Five Nights at Freddy's games. It was first used in Five Nights at Thomas's.

Function
The Furnace is used to recharge the Power the player has.

Furnace (Five Nights at Thomas's)
The furnace is inside the room, clicking on it results in a short cutscene where a cylinder (presumably an arm) reaches into the furnace, after which the player's amount of Power (In this case, Coal) increases-

Digital Furnace (Five Nights at Thomas's 2)
This time the player has to open the Monitor to use the furnace.

Dorito Power (Five Nights at Illuminati)
Similar to the Furnace in Five Nights at Thomas's, this sits in the office. However, the cooldown time is a lot longer.