38 is biased e g spring biased to select the first outlet port 42 of the three way valve 38 when the evaporator coil 52 requires defrosting and when heating is required valve 92 is energized and the low pressure side of the compressor 22 operates the three way valve 38 to select the second outlet port 44 to begin operation in the heating mode and or defrost modes a condenser fan or blower 104 directs ambient air represented by arrows 106 across the condenser coil 52 return air represented by arrows 108 heated by contact with the condenser fan 104 is discharged to the atmosphere an evaporator fan 110 draws load space air represented by arrows 112 through an inlet 114 in a bulkhead or wall 116 and upwardly through conduit 118 a return air temperature sensor 120 measures the temperature of air entering the inlet 114 an evaporator coil temperature sensor 136 can be positioned adjacent to or on the evaporator coil 68 for recording the evaporator coil temperature in other embodiments the evaporator coil temperature sensor 136 can be positioned in other locations in still other embodiments other sensors such as for example the return air temperature sensor 120 and or the discharge air temperature sensor described below 126 can also or alternately be used to calculate the evaporator coil temperature discharge air represented by arrow 122 is returned to the load space 14 via outlet 124 discharge air temperature sensor 126 is positioned adjacent to the outlet 124 and measures the discharge air temperature during the defrost.