|Fig. 1: Heat pump refrigeration cycle. (Source: Wikimedia Commons)|
Heat pumps have been heating and cooling houses for some time now. Efficiency and cost effectiveness of these devices have increased dramatically since they were first used for domestic heating and cooling. With environmental and financial needs to decrease energy consumption, heat pumps have improved even more. Heat pumps are used for air conditioning as well as water heating because they can move energy into and out of a system. When cooling a house, it extracts heat from the air in the house, yielding a cooler atmosphere inside of the house. However, the heat taken from the house is wasted into the outdoor air. This wasted heat can be used to heat incoming water that needs to be heated for showers, laundry machines, etc. 
To understand how to incorporate air conditioning with water heating, we must first see how heat pumps operate. Following the heat pump refrigeration cycle in Fig. 1, we begin at the cool side. Cool refrigerant is then directed to a compressor which increases the pressure and temperature of the liquid. The liquid then passes through a heat exchanger which condenses the liquid and warms the material on the other side of the heat exchanger. Condensation is an exothermic process that decreases the temperature of the refrigerant. The refrigerant is then directed to an expansion valve which decreases the pressure and temperature of the liquid. The liquid passes through another heat exchanger which evaporates the refrigerant and cools the material on the other side of the heat exchanger. Evaporation is an endothermic process that increases the temperature of the refrigerant. This cycle runs continuously to create a hot side and a cold side of the heat pump. 
Now that the basics of heat pumps are understood, we can apply it to domestic heating and cooling. In order to combine an air conditioner and water heater, only one heat pump must be used. All matters of cooling will be attached to the heat exchanger on the cool side, and all matters of heating will be attached to the heat exchanger on the hot side. This design works well when heating and cooling needs are about equal.  This is the case in tropical and subtropical regions due to the fact that these regions have no need for space heating.  If the heat pump continuously cools the house while heating water, energy consumption is effectively cut in half. The heat that would be wasted from cooling the house is all used to heat water. The heat pump can continuously be set to this specification and may essentially move heat from the air in the house to the water. Consumers of this integrated heat pump may essentially enjoy free hot water. 
© Nick Davidson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Techarungpaisan, S. Theerakulpisut, and S. Priprem, "Modeling of a Split Type Air Conditioner With Integrated Water Heater," Energy Convers. Manage. 48, 1222 (2007).
 J. Ji et al., "Domestic Air-Conditioner and Integrated Water Heater For Subtropical Climate," Appl. Therm. Eng. 23, 581 (2003).