hypocaust

/HH AY0 P AH0 K AO0 S T/
noun
  1. 1

    An underfloor space or flue through which heat from a furnace passes to heat the floor of a room or a bath.

  2. 2

    An underfloor heating system, even without such an underfloor space or flue, as adapted for modern housing.

Source: https://en.wiktionary.org/wiki/hypocaust