Abstract: An accumulator distillation insert is provided for allowing the composition f a zeotropic refrigerant to be adjusted based upon a building heat load. The accumulator distillation insert comprises an accumulator, the accumulator having a two-phase input and a vapor output, the accumulator having a refrigerant mixture, in liquid form, disposed at a bottom of the accumulator; an immersion heater is provided in communication with the refrigerant mixture, in liquid form; and a distillation column is disposed in the accumulator, the distillation column is maintained above the refrigerant mixture in liquid form, the distillation column is provided for changing the chemical composition of the refrigerant mixture by decreasing the amount of volatile components in the refrigerant mixture, stored in the accumulator and thereby increasing the capacity of the heat pump system.