For many years. indoor heat was provided bywood or coal burning fireplaces, kitchen stoves and pot belly stoves.  Eventually kerosene heaters became popular.  They were somewhat dangerous because of fire risk and fumes, particularly from non-ventilated heaters.  As gas became available in homes, heating units in fireplaces became more popular.  The unit had a ceramic area that radiated heat.  Fumes from the unit rose up into the fireplace chimney.