Some heat pumps have been designed to operate utilizing a water source instead of an air source simply by designing the outdoor heat exchanger to operate between the heat pump working fluid and water instead of between the working fluid and air. These so-called water-to-air heat pumps have advantages over the air-to-air type if a relatively warm source of water is available which does not require an excessively large amount of pumping power. In particular, industrial waste heat might be used.
In this case, the difference from the water-to-water heat pump is in the method of treating the air. This system provides heating and cooling of air with water as the heat sink or source. The same sources of water can be used in these systems. These systems are less efficient than the water-to-water systems because of the much lower heat transfer coefficient of air. These systems are commonly used in large buildings and sometimes in industrial applications to provide hot or cold water.
The water-to-air heat pumps removes heat from water and converts it to hot air in exactly the same way that a cold water drinking fountain removes the heat from the water and discharges the heat from the side or back of the drinking fountain.