A model was used to calculate the boiler efficiency (and further the fuel consumption) on a monthly basis using the hydraulic energy values supplied by the boiler to the domestic hot water tank and to the space heating system. The main idea of this model is to sum up the "Nominal standby heat loss energy” corrected by two factors. Factor one estimate the standby time per month and is simply calculated by the hours of the month minus the hours of running the boiler on nominal power for supplying the necessary energy. Factor two estimates how big are the stand by losses of a boiler depending on the thermal mass and the approx. number of starts per day. Very heavy boilers have a factor from 0.7 to 1.0, very light boilers have a factor of 0.2 to 0.5. Detailed descriptions to this model can be found in [2].

In Fig 8, the calculated and the measured fuel consumption is compared. In the case of the condensing natural gas burner the constant value of 0.2 was used as factor two for all months. The maximum differences in the monthly fuel consumptions were -2% (Feb) and +6% (Jun). On a yearly basis the difference between measured and calculated values was -0.1%. In the case of the oil boiler a constant value of 1 was used as factor two for all
months. The maximum differences in the monthly fuel consumptions were -2% (Jul) and +5% (May). On a yearly basis the difference between measured and calculated values was +2%. This shows that with this calculation model on a monthly basis, the results in calculated fuel consumption are very close to the measured consumptions.

In combination with this model of calculating the fuel consumption, a simulation model was set up with the simulation program

SHWwin [9]. The boundary conditions of this model are the same as the house with the monitored condensing gas burner. The measured monthly domestic hot water (dhw) load-profile was used as input to the model. The model of the building which generates the space heating (sh) demand was fitted to the measurements by adapting typical parameters (e. g. maximum heat load, room temperature, reduced temperature during a specified period in the night, dimensions of windows regulating the passiv solar gains, etc). The Danish Test Reference Year was used as input for the simulations. In Fig 9 the simulation results and the calculations of the fuel consumption compared with the measured fuel consumption in the case of the condensing natural gas burner project are shown.

The demand for domestic hot water and hot water tank losses fits very good as it is easily possible to adjust the parameters for that. The difference between measured and calculated space heating demand varies from month to month because of the different climate of the test reference year instead of the measured weather data. Consequently also the fuel consumption varies in the same way. On a yearly basis the space heating demand of the simulation model is 0.4 % lower than measured. The demand on domestic hot water plus hot water tank losses is 0.2 % higher than measured. So the yearly result of

3.6 % higher calculated fuel consumption than measured fuel consumption can be interpreted as a very good result. With this verified model the following simulations were done to find out the saving potentials on fuel consumption with solar heating systems.