Defrost control

A method of controlling when to initiate a defrost cycle to remove accumulated frost from an outdoor heat exchanger coil forming a portion of a refrigerant heat pump system including a compressor, said method comprising the steps of:

• sensing a value of ambient temperature of the outdoor heat exchanger;
• sensing the temperature value of the refrigerant in the outdoor heat exchanger;
• defining a two dimensional coordinate system wherein a first coordinate corresponds to ambient temperature of the outdoor heat exchanger and wherein a second coordinate corresponds to the refrigerant temperature in the outdoor heat exchanger;
• defining regions of points having coordinate values relative to the two dimensional coordinate system, the regions including a first region of points wherein a first conditionally activated defrost action is to occur and a second region wherein a second conditionally activated defrost action is to occur, and a third region wherein no defrost action is to occur;
• defining a particular point in space relative to the two dimensional coordinate system, the point having a first coordinate value corresponding to the sensed value of ambient temperature of the outdoor heat exchanger and a second coordinate value corresponding to the sensed value of the refrigerant temperature in the outdoor heat exchanger;
• determining whether the particular point in space lies within the first, second or third regions;
• and
• implementing a first conditionally activated defrost action if the point lies in the first region and implementing a second conditionally activated defrost action if the point lies in the second region.