In the last fifty years, several large office buildings and small college campuses have been constructed using water-to-water heat pumps. Their capacities were up to several hundred tons. These systems usually use well water. That means two wells are used. One is used for supply and one for disposal. A possible arrangement is shown in Fig. 17-23.
The supply and disposal wells are manually selected. Well water and return water are mixed, for both evaporator and condenser. This is done on a temperature basis. Under some conditions, this system can become an internal source heat pump. That is, when the exterior-zone heating and interior-zone cooling loads are in balance, or nearly so, little or no well water is needed.
Internal source heat pumps without wells are used where there is sufficient internal cooling load to supply the net heating requirements under all conditions. Excess heat can be disposed of through cooling towers.
A problem with these systems is related to a high electrical load for the pumping system. A variety of variable-flow piping schemes have been devised to overcome this problem.