The nuclear furnace room was a location found on Federation starships, in particular Constitution-class vessels outfitted as heavy cruisers, the location of the ship's nuclear furnace.
History and specifics[]
In the year 2266, James T. Kirk warned Spock and Montgomery Scott that the next one to start fighting again would be wiping the pipes in the nuclear furnace room for a month. (TOS comic: "The Cosmic Cavemen")