. Conclusions
A mathematical model for the evaluation of the seasonal efficiency of heating systems based on electric air-towater
heat pumps integrated by electric heaters is used in this paper to analyze the seasonal coefficient of
performance, SCOP, as a function of the climate, of the building thermal needs and of the heat pump type (on-off
heat pump (ON-OFF HP), multi-compressor heat pump (MCHP) or inverter-driven heat pump (IDHP)). The
numerical results highlight that, to enhance the seasonal efficiency of the system, there exists an optimal value of the
bivalent temperature, which is linked to the choice of the heat pump size with respect to the building. The numerical
results show that IDHPs allow to reach the highest values of SCOP and the optimal sizing of the IDHP is obtained
when the bivalent temperature is selected as equal to the design temperature. Also in the case of MCHP and ONOFF
HP there exists an optimal value of the bivalent temperature which maximizes the system performance, but it is
larger than the design temperature.