To be able to calculate the thermal performance of one specific thermosiphon DHW system, it is necessary to identify the surface that describes the thermal behaviour of the system in the best way. For the selection of the best fitting surface, the result of at least one DST test per product line is needed. In Table 2, an example for a product line of six different DHW systems is given. The solar fractions of system 1, 2 and 6 have been determined based on measurements according to the DST method and on a long term performance prediction. The solar fractions of system 3, 4 and 5 are not known a priori and have to be calculated by the extrapolation procedure. The best fitting surfaces is defined as the surface where the discrepancies between the values offsol obtained by the DST method and obtained with equation (2) are minimum. These values are compared with the fSol-values that have been derived directly for the DST measurements. The differences Afsol, between measured and predicted values is calculated with equation (3),

sol, i, j sol, measured sol, calc, i, j

with i = 1…3: system number, j = 1….12: surface number.

The total difference AFsol is defined as the summation over the differences Afsoi? i

AF, o =Z|A«,u

i

It is assumed that the surface where the value of AFsol is minimal describes the system behaviour in the best way. This function will then be used to compute the solar fraction for the remaining systems 3, 4 and 5. Table 3 lists the results determined by the mathematical extrapolation procedure according to equation (3) for the solar fractions of systems 3, 4 and 5.