PURPOSE:To prevent the mixing of the hot water and cold water in a tank used for a system to heat a greenhouse for agriculture or fishery, or the like, and enable a single tank to serve as both a hot-water tank and a cold-water tank, by providing a hot-water outlet in the upper part, together with both a cold-water inlet and a water flow-settling plate in the lower part of the tank. CONSTITUTION:A hot-water outlet nozzle 12 is provided in the upper part of a circulating-water tank 11, while a cold-water outlet nozzle 13 is connected to a pump inlet piping 3E through a piping as well as to a cold-water return piping 3D so as to function also as a cold-water inlet nozzle. Moreover, a flow-settling plate 14 is attached at the bottom part of the circulating-water tank 11. Said plate 14 has a porous plate shape provided with a plurality of through holes and is welded to the flange attached to the inner circumference of the circulating-water tank 11. By the circulating-water tank thus constituted, the mixing of the hot water and cold water in said tank can be prevented.
