Water-to-water geothermal heat pumps heat and cool water, for hydronic radiant systems, or for dedicated water heating in commercial buildings (for example, washing machines in a Laundromat or hospital). If you are interested in radiant in floor heating, water-to-water is for you. This system has two water loop systems. One is connected to the earth loop and the other is connected to the house for heating.
Water-to-air geothermal heat pumps heat and cool air, for forced-air ducting systems. In the United States, most homes use forced-air ducting distribution systems, so water-to-air units are the standard type used. There is one water loop connected to the earth, through a coil the heated or cooled air is transferred to by duct to the house.
Is a combination system that primarily uses a Water-to-water system that utilizes a water coil and duct to heat or cool a part of the home while relying on radiant on the other part of the home.