In this paper the heating system return temperature effect on condensing temperature and the seasonal performance factor of the heat pump was measured and quantified with analytical model, IDA-ESBO detailed simulation with heat pump model and by correlation equation derived from laboratory measurements. Results show that the.calculation with derived condensing temperature correlation stressed the effect of relatively low return temperature in a radiator heating system and provided about 9% higher SPF value, compared to analytical and IDA-ESBO model. Four studied heat pump connection schemes resulted in a range of SPF values of 3.72-4.21, where the highest SPF was achieved with direct connection of heat pump and heating system, which had the lowest return water temperature. Additionally a hand calculation based on hourly heating energy needs and estimated return temperature was tested. This estimation resulted in 4.5% lower SPF value, showing the importance to use correct return temperature corresponding to actual flow rates in radiators. The results of this study indicate that calculation procedures, which do not calculate the return temperature and use simple assumptions in the condensing temperature calculation may lead to underestimation of SPF by more than 10% in the case of radiator heating. (C) 2015 Elsevier B.V. All rights reserved.