This article discusses how heat pump systems can be used to heat water in homes in Sweden. The researchers looked at different ways to heat water efficiently, as it requires higher temperatures than heating the house. They found that storing hot water is important to meet the high demand for hot water. Using natural convection to heat water can be effective, but it may cause mixing issues. Storing water in a closed loop system can reduce the risk of bacteria growth. Heating water in an external heat exchanger can also help prevent mixing problems. The researchers also found that there is limited data on water usage in single-family homes compared to apartment buildings.