Abstract: A heating system that heats a structure includes a variable speed compressor, an outdoor air temperature (OAT) sensor, and a saturated suction temperature (SST) sensor. A processor connected to the variable speed compressor, OAT sensor, and SST sensor electronically receives a demand to heat the structure and causes the variable speed compressor to operate at a speed determined by the demand. After some time, a current defrost value is determined from the difference between the current OAT and current SST. The current defrost value is compared to a target defrost value that is determined using an initial defrost value, a defrost speed, and the selected speed. When the current defrost value exceeds the target defrost value, the processor causes the heating system to begin performing a defrost cycle by switching a reversing valve and operating the variable speed compressor at a defrost speed.