A furnace was a device that produced enough heat energy to give a building warmth.
In 1930, Spock was fixing the furnace at the Twenty-First Street Mission. When he informed James T. Kirk that he was going to finish doing so, Edith Keeler noticed that the Vulcan always seemed to be calling Kirk "Captain", even when he didn't say the word. (TOS: "The City on the Edge of Forever ")