hy·po·caust 
(hīpə-kôst′)
(hīpə-kôst′)n.
A space under the floor of an ancient Roman building through which flue gases from a furnace were passed to heat a room or a bath.
[Latin hypocaustum, from Greek hupokauston, from hupokaiein, to light a fire beneath : hupo-, hypo- + kaiein, to burn.]