A method for preventing a frosted evaporator heat exchanger coil of a refrigerant vapor compression system for supplying conditioned air to a temperature controlled space, the method characterised by comprising: determining whether a change in an air flow temperature differential across the evaporator heat exchanger coil over a preselected period of time is greater than or equal to a set point threshold change in air flow temperature differential; determining whether a change in an evaporator heat exchanger coil refrigerant pressure condition over said preselected period of time is greater than or equal to a set point threshold change in refrigerant pressure condition; if both the change in air flow temperature differential across the evaporator heat exchanger coil over a preselected period of time is greater than or equal to the set point threshold change in air flow temperature differential and the change in the evaporator heat exchanger coil refrigerant pressure condition over said preselected period of time is greater than or equal to the set point threshold change in refrigerant pressure condition, determining whether a current magnitude of the air flow temperature differential across the evaporator heat exchanger coil is greater than or equal to a set point threshold magnitude for the air flow temperature differential, and determining whether a current magnitude of the evaporator heat exchanger coil refrigerant pressure condition is less than or equal to a set point threshold magnitude for the refrigerant pressure condition; and initiating a defrost of the evaporator heat exchanger coil if both the current magnitude of the air flow temperature differential across the evaporator heat exchanger coil is greater than or equal to the set point threshold magnitude for the air flow temperature differential, and the current magnitude of the evaporator heat exchanger coil refrigerant pressure condition is less than or equal to the set point threshold magnitude for the refrigerant pressure condition. determining whether a change in an air flow temperature differential across the evaporator heat exchanger coil over a preselected period of time is greater than or equal to a set point threshold change in air flow temperature differential; determining whether a change in an evaporator heat exchanger coil refrigerant pressure condition over said preselected period of time is greater than or equal to a set point threshold change in refrigerant pressure condition; if both the change in air flow temperature differential across the evaporator heat exchanger coil over a preselected period of time is greater than or equal to the set point threshold change in air flow temperature differential and the change in the evaporator heat exchanger coil refrigerant pressure condition over said preselected period of time is greater than or equal to the set point threshold change in refrigerant pressure condition, determining whether a current magnitude of the air flow temperature differential across the evaporator heat exchanger coil is greater than or equal to a set point threshold magnitude for the air flow temperature differential, and determining whether a current magnitude of the evaporator heat exchanger coil refrigerant pressure condition is less than or equal to a set point threshold magnitude for the refrigerant pressure condition; and initiating a defrost of the evaporator heat exchanger coil if both the current magnitude of the air flow temperature differential across the evaporator heat exchanger coil is greater than or equal to the set point threshold magnitude for the air flow temperature differential, and the current magnitude of the evaporator heat exchanger coil refrigerant pressure condition is less than or equal to the set point threshold magnitude for the refrigerant pressure condition.