US 5257506 A abstract a defrost cycle for a heat pump system which optimizes the efficiency of the heat pump by initiating defrost depending upon the relationship of both the outdoor coil ambient temperature and the outdoor coil refrigerant temperature with a predetermined temperature reference level application_number US 67344891 A description referring now to fig1 there is shown a heat pump system 10 comprising an indoor coil 11 and outdoor coil 12 a compressor 13 and a reversing valve 14 installed in the line 15 between the indoor and outdoor coils 11 and 12 are expansion valves 16 and 17 with each having provision for bypassing refrigerant when it is not acting as an expansion device all of these components operate in a rather conventional heat pump manner to provide cooling to the indoor space while operating in the air conditioning mode and heating to the indoor space while operating in a heating mode although the present invention is equally applicable to either constant speed or variable speed systems it will presently be described with reference to a constant speed system such a system contemplates the use of multi speed motors such as for example a two speed compressor motor the motor 33 drives the compressor 13 which is normally located in the outdoor section near the outdoor coil 12 the motor 37 drives the fan 27 for the indoor coil 11 and the motor 34 drives the outdoor fan 24 a compressor controller 18 is therefore provided to communicate with and to coordinate the operation of the compressor and its associated equipment the.