In total four rather different systems have been investigated, two systems with a pellet stove (system 1 and 2), one with a store integrated pellet burner (system 3) and one with a pellet boiler (system 4) (see also figure 2 and table 1).

System 1 is the simplest system using separate units to provide domestic hot water and space heating. A pellet stove transfers the heat to the building by convection and radiation. This requires a building with open interior design in order to allow a good heat distribution to the building. The power of the stove is automatically and continuously modulated according to the room temperature, but has a limiting minimal power. In the specified power operation range between 2 and 6 kW the stove reaches efficiencies between 83 and 89 % under stationary conditions (Persson, 2004). The domestic hot water is provided by a solar hot water system comprising a 280 liter store and 5m2 of solar collectors. The solar circuit is coupled to the storage by an immersed heat exchanger in the bottom of the store. The auxiliary heat is provided by a electrical heater in the top third of the store.

Figure 1. Simulated performance of the water mantled stove in system 2 with constant room temperature of 22°C, water return temperature of 55°C, mass flow of 0,111 kg/s and air factor of 6

(Pmin) and 4 (Pmax).

System 2 is rather similar to system 1 but the pellet stove delivers heat to the building in two ways: directly by convection and radiation as in system 1; and indirectly through an inbuilt heat exchanger to the water based radiator system. Approximately 80% of the produced heat can be transferred to the radiator system
when the stove is operated under stationary conditions with the maximum combustion power (figure 1). The stove is on/off controlled by the room temperature, operating by default with the maximum power.

System 3 is a solar combisystem with a store integrated pellet burner and a water based radiator system. All required heat for hot water and space heating is taken from the combistore, the water for space heating directly and the domestic hot water by two immersed heat exchangers placed in the bottom and the top of the combistore. The heat from the solar heating circuit is transferred by another immersed heat exchanger to the bottom of the store. The store integrated pellet burner delivers heat by a water to air heat exchanger consisting of horizontal pipes in the upper part of the store. The burner is on/off controlled by a sensor placed in the storage tank above the burner. The pellet burner has a maximum power of 25 kW, enough capacity even for single family houses with a rather high space heating demand. The burner can be adjusted for summer operation to half of the maximum combustion power. The collector area for system 3 and 4 is about 10 m2 which somewhat typical for Swedish solar combisystems.

System 4 is also a combisystem but uses an external pellet boiler as the main auxiliary heat source. The pellet boiler is coupled to the store by another immersed heat exchanger in the upper part of the store. The boiler is on/off controlled and has an internal water volume of 140 liter. The boiler contains an integrated heat exchangers for hot water preparation, but his was not used. No connections are available to couple the boiler to a solar circuit. Consequently only the space heating part of the boiler was used and connected to a combistore.