An open-loop, ground-water heat pump, uses a surface or underground water source (such as a lake, river, or well) as the heat source and sink. Well water designs are the most common and seem to be the most cost effective. The well supplies both domestic water and water for the heat pump. Approximately three gallons per minute of well water are needed per ton of cooling capacity.

Ground water source open-loop heat pumps use the same concept as the ground coupled units - for example, in the Midwest the temperature of the earth near the surface and the water in it (aquifer) is typically around 55°F. Water is taken from the ground or surface water (pond, lake, etc.), circulated to the individual heat pumps and the returned to the ground via a disposal well, returned to the lake or pond, or where permitted discharged into a stream or river.

When more units are heating than cooling the circulating water temperature drops prior to disposal. Conversely, when more units are cooling than heating, the circulating water is warmed prior to disposal.

Advantages

In both commercial and residential installations, geothermal heat pump systems typically have lower maintenance costs than conventional systems as all equipment is installed inside the building or underground. This means that there is no outside equipment exposed to weather and vandalism. All refrigerant systems are sealed, similar to household refrigerators.

Open loop systems have less loss in heat transfer
Open loop systems have lower heat pump energy costs
Geothermal systems are very flexible. They can be easily and inexpensively subdivided or expanded to fit building remodeling or additions. They are particularly well-suited to "tenant finish" installations.

In commercial installations, systems can save money by recovering excess heat from building interior zones and moving it to the perimeter of the building. They can also save money by allowing management to isolate and shut down unoccupied areas of the building.

Disadvantages

Problems associated with disposal of the water after once-through the heat pump, disposal wells can be costly,
Water which has suitable qualities could change with time to poor quality that causes problems of corrosion later in time,
Costs associated with the drilling of the water well can be high and unknown at the outset,
Water tables and well output can change over time and cause future problems,
Permitting and its costs are usually required,
Open-loop systems have more potential problems than either conventional systems or closed-loop geothermal systems because they bring outside water into the unit. This can lead to clogging, mineral deposits, and corrosion in the system.
Open-loop systems require a large supply of clean water in order to be cost effective. This often limits their use to coastal areas, and areas adjacent to lakes, rivers, streams, etc. In addition, there must be an acceptable method of returning the used water to the environment. This may be limited not only by environmental factors (such as no place to dump that much water), but also by local and state regulations.
Since accessibility to terminal units is important in geothermal systems, architects and mechanical and structural designers must carefully coordinate their work.
Each unit requires both electrical and plumbing service.
Duct systems must be installed to bring outside air to each space.
Secondary or backup heat sources are required in cooler climates.