A heat pump typically initiates its automatic defrost mode when outdoor ambient temperatures drop below 35 to 40 degrees Fahrenheit (roughly 1.5 to 4.5 degrees Celsius) and high outdoor humidity causes moisture in the air to condense and freeze onto the cold outdoor coil fins. The defrost cycle is not triggered by a specific calendar temperature alone, but rather by a combination of outdoor coil temperature sensors and a timer on the defrost control board that measures how long the compressor has run. When frost buildup restricts airflow and lowers the coil temperature to a preset threshold, the board shifts the system into a temporary cooling mode to melt the ice. You can verify that the defrost cycle is functioning normally when you see steam rising from the outdoor unit and notice ice disappearing within ten to fifteen minutes.
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