In these heat pump systems, the heat source and the heat sink are water. The heat pump system takes heat from a water source (by coil A) while simultaneously rejecting it to a water heat sink (by coil B) and either heats or cools a space or a process. In practice, there are many sources of water, e.g. waste water, single or double well, lake, pool, and cooling tower. These heat pumps use less electricity than other heat pumps when they are properly maintained. However, without proper maintenance the operating costs increase dramatically.
Table 4.5 shows typical COPs for a water-to-water heat pump operating in various heat distribution systems. The temperature of the heat source is 5°C, and the heat pump Carnot efficiency is 50%.