For combisystems, the heat store is the key component, since it is used as short-term storage for solar energy and as buffer storage for the fuel or wood boiler. The storage medium used in solar combistores is usually the water of the space heating loop and not the tap water used in conventional solar domestic hot water stores. The tap water is heated on demand by passing through a heat exchanger, which can be placed either inside or outside the tank containing the water of the heating loop. When the heat exchanger is in direct contact with the storage medium, the maximum tap water temperature at the start of the draw-off is similar to the temperature of the water inside the store. The tap water volume inside the heat exchanger can vary from a few liters for immersed heat exchangers to several hundred liters for tank-in-tank stores.