Lewis & Short

praefurnĭum, i, n. [prae-furnus].

  1. I. The opening of a furnace, the door by which fuel is admitted: si uno praefurnio coques, Cato, R. R. 38, 1.
  2. II. The heating-room, or hot anteroom of a bath, = προπνιγεῖον, Vitr. 5, 10, 2; 7, 10.