The four basic heat pump designs for space heating and cooling employ:
• air as the heat source-sink and air as the heating and cooling medium,
• air as the heat source-sink and water as the heating and cooling medium,
• water as the heat source-sink and air as the heating and cooling medium, or
• water as heat source-sink and water as the heating and cooling medium.
Each of these basic designs can supply the required heating and cooling effect by changing the direction of the refrigerant flow, or by maintaining a fixed refrigerant circuit and changing the direction of the heat source-sink media. A third alternative is to incorporate an intermediate transfer fluid in the design. In this case the direction of the fluid is changed to obtain heating or cooling and both the refrigerant and heat source-sink circuits are fixed. The fixed refrigerant circuit designs, generally referred to as the indirect type of application, are becoming increasingly popular, particularly in the larger capacities.