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. Provisions are also made for water disposal, such as a return well.

Depending on the application, location, and groundwater inlet temperature to the heat pump, approximately 1½ to 3 gallons per minute of source water (well, lake, etc.) are needed per ton of cooling capacity. A 3,000 square foot, well-insulated home would typically require 8 to 15 gallons per minute. The best flow rate is an optimization of pump power vs. compressor power.