During a defrost cycle, the heat pump switches from heating to cooling. To prevent cool air from being circulated when heating is needed, the control automatically turns on the furnace to compensate for the heat pump defrost cycle. (Most modern heat pump systems do the same thing with strip heating.) When supply air temperature climbs above 110 to 120°F (43.3 to 48.9°C), the defrost limit control turns off the furnace and keeps indoor air from getting too warm.
After a defrost cycle, the air temperature downstream of the coil may go above the 115°F (46.1°C) closing point of the heat pump delay. Then, the compressor will stop until the heat exchanger has cooled to 90 to 100°F (32.2 to 37.8°C), as it does during normal cycling operation between furnace and heat pump.