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Providing heat pumps for the retrofit of fuel based boilers in residential heating, is a short or medium term challenge and a major opportunity for a substantial reduction of greenhouse gas emissions. While effective solutions for improved high temperature lift, vapor compression heat pumps have been analyzed elsewhere, this paper specifically deals with the refrigerant selection aspects for alternative blends, using a multi-objective optimization algorithm. The objectives COP and specific heat output are calculated for arbitrarily mixed refrigerant blends, using 17 selected pure fluids. Seasonal mean operating conditions (ambient air 0°C, heating water 50°C) have been used to simulate the heat pump cycle. Blends with a saturation curve extending over the range of –20°C (normal boiling point) and 70°C (critical point) are considered. Alternative blends are compared with the commercial mixture R-407C. The refrigerant mixture with the highest COP shows an improvement of 8% on the low capacity end of the Pareto-optimum curve. By maximizing the specific heat rate, a gain of 83% can be achieved. All the dominating solutions are composed of flammable refrigerants.