Four main types of heat losses have been studied in this article. Flue gas losses are mainly determined by the construction of the pellet burner, the area of the heat exchanger, the dimensions of the flue gas passages etc. Parameters such as combustion power and the surplus air influence strongly the flues gas losses. Flue gas losses are considered to be leakage losses when the burner and the fan have stopped, after the stop phase of combustion has been completed. In order to prevent confusion the flue gas losses are defined as the total flue gas losses minus the leakage losses.

Qfl = Qfltot — Qleak (3)

For leakage losses, the air mass flow through the burner is calculated by type 210 as follows:

T — T

ob outd

50

where m’g50 is the flue gas mass flow at 50 K temperature difference between the gas leaving the burner (Tob) and the outdoor temperature (Toutd). The total leakage losses depend strongly on the thermal mass of the stove or boiler.

The store heat losses depend on the quality and tightness of the store insulation, the store envelope area and the temperature difference between the store content and the ambient. The UA-values for the stores that have been used in the simulations can be found in table 1. The values for the hot water stores in system 1 and 2 are based on theoretical calculation, but also match well with UA-values from measurements for rather well insulated DHW-stores (Vogelsanger, 2004). The combistore of system 3 has been

tested at SERC and two mantle UA-values, one for the top part where the pellet burner is located and one for the bottom part, have been determined. Also the UA-values for the top and the bottom of the tank have been obtained from measurements and theoretical calculations. The UA-values for the combistore of system 4 are given by Bales (2003). The boiler/burner heat losses are obtained from an output of type 210 calculated based on identified heat loss coefficients.