PURPOSE: To have safety operation of a pump by returning part of, or a small amount of, the hot water discharged onto the ground back to the hot water takeout well through a warming piping, thereby keeping the intra-well temp. evenly, and by securing the pump with necessary suction pressure.

CONSTITUTION: During operation, part of, or a small amount of, the hot water is returned to the gaseous phase 30 of the well from the portion of discharge pipe 15 located on the ground through a by-pass piping 33 by opening a gate valve 31 upon adjustment of its rate of flow with a flow adjusting valve 32. Because, at said gaseous phase 30 of well, the surrounding soil and rock ground will absorb the heat, the temp. tends to sink to show the saturated steam pressure for the current condition, so that any significant temp. drop will cause too low pressure to lead to disability of a hot water pump 12 with its suction function. In respect thereof, the above-mentioned by-pass piping 33 gives said gaseous phase always a minute amount of hot water at high temp., so that the temp. of the gaseous phase 30, is raised to provide a corresponding pressure. Thus the hot water pump 12 can make safety operation.