means of the recirculation of water in the pipe 48 carrying heat to radiator 55 during winter evenings to provide additional thermal insulation at the window 12 to prevent heat leaking from the stack 24 therethrough a removeable exterior thermally insulating and opaque cover or curtain 57 may be provided over the window additionally during summer days the window may be covered with cover 57 to maintain the stack in a somewhat cool condition and the water circulation within pipe 48 may be utilized to cool the building by taking in heat energy at the radiator 55 and transferring it to the stack 24 as a further feature the radiator 55 might be replaced with a water tank not shown within the building and the apparatus of fig2 could be utilized as a hot water heater it will be appreciated that air trapped within the toroidal interior of each tire 14 provides an effective thermal insulating barrier against heat transfer between the rocks 45 or other energy storage media within the cylindrical interior of the stack 24 and the air space surrounding the stack in the embodiment illustrated in fig5 and 6 the stack 24 is set upon a planar mesh screen or apertured plate 59 which rests upon blocks 44 a hot air duct 36 is directed.