What triggers the defrost cycle on a heat pump?

The defrost cycle on a residential air-source heat pump is triggered by a combination of time and temperature parameters monitored continuously by the defrost control board and outdoor coil temperature sensors. When the heat pump operates in heating mode during cold weather, moisture from the ambient air condenses and freezes on the cold outdoor coil fins. The defrost board uses a timer—typically set to 30, 60, or 90 minutes of compressor runtime—to check whether the outdoor coil temperature sensor has dropped below a specific threshold (usually around 30 to 35 degrees Fahrenheit). If both the time interval has elapsed and the coil temperature sensor confirms frost accumulation, the control board initiates the defrost cycle. You can verify that the trigger mechanism is working properly by observing the system shift into defrost mode and noting that ice accumulation begins to melt away from the outdoor coils.