A method of optimizing a heating capacity of a vapor compression system comprising the steps of: pumping a fluid through a first heat exchanger (24, 32) with a pumping device; exchanging heat between a refrigerant flowing through the first heat exchanger (24,32) and the fluid; exchanging heat between the refrigerant flowing through a second heat exchanger (28, 46) and an outdoor fluid medium; compressing the refrigerant to a high pressure; cooling the refrigerant in the step of exchanging heat between the refrigerant flowing through the first heat exchanger (24,32) and the fluid; expanding the refrigerant to a low pressure in an expansion device; evaporating the refrigerant in the step of exchanging heat between the refrigerant flowing through the second heat exchanger (28,46) and an outdoor fluid medium, and evaporating the refrigerant includes accepting heat from the outdoor fluid medium; sensing a temperature of the outdoor fluid medium; characterized by either: a) detecting a current supplied to the pumping device and determining the maximum current supplied to the pumping device, or b) detecting three measured characteristics including an ambient temperature, a fluid inlet temperature of the fluid entering the first heat exchanger (24,32), and a fluid outlet temperature of the fluid exiting the first exchanger (24,32), the first heat exchanger (24, 32) being a heat rejection heat exchanger; optimizing the heating capacity when the step of sensing a temperature of the outdoor fluid medium determines that the temperature is below a threshold value by determining an optimal heating capacity pressure (P 2) based on either: a) a correlation of the maximum current supplied to the pumping device to the optimal heating capacity pressure (P 2); or b) the three measured characteristics; and adjusting a high side pressure of the vapor compression system to the optimal heating capacity pressure (P 2). pumping a fluid through a first heat exchanger (24, 32) with a pumping device; exchanging heat between a refrigerant flowing through the first heat exchanger (24,32) and the fluid; exchanging heat between the refrigerant flowing through a second heat exchanger (28, 46) and an outdoor fluid medium; compressing the refrigerant to a high pressure; cooling the refrigerant in the step of exchanging heat between the refrigerant flowing through the first heat exchanger (24,32) and the fluid; expanding the refrigerant to a low pressure in an expansion device; evaporating the refrigerant in the step of exchanging heat between the refrigerant flowing through the second heat exchanger (28,46) and an outdoor fluid medium, and evaporating the refrigerant includes accepting heat from the outdoor fluid medium; sensing a temperature of the outdoor fluid medium; characterized by either: a) detecting a current supplied to the pumping device and determining the maximum current supplied to the pumping device, or b) detecting three measured characteristics including an ambient temperature, a fluid inlet temperature of the fluid entering the first heat exchanger (24,32), and a fluid outlet temperature of the fluid exiting the first exchanger (24,32), the first heat exchanger (24, 32) being a heat rejection heat exchanger; a) detecting a current supplied to the pumping device and determining the maximum current supplied to the pumping device, or b) detecting three measured characteristics including an ambient temperature, a fluid inlet temperature of the fluid entering the first heat exchanger (24,32), and a fluid outlet temperature of the fluid exiting the first exchanger (24,32), the first heat exchanger (24, 32) being a heat rejection heat exchanger; optimizing the heating capacity when the step of sensing a temperature of the outdoor fluid medium determines that the temperature is below a threshold value by determining an optimal heating capacity pressure (P 2) based on either: a) a correlation of the maximum current supplied to the pumping device to the optimal heating capacity pressure (P 2); or b) the three measured characteristics; and a) a correlation of the maximum current supplied to the pumping device to the optimal heating capacity pressure (P 2); or b) the three measured characteristics; and adjusting a high side pressure of the vapor compression system to the optimal heating capacity pressure (P 2).