When outdoor temperatures drop below freezing, an air-source heat pump continues to extract thermal energy from the ambient air, though its heating efficiency naturally decreases. Even in freezing weather, air contains usable heat energy down to absolute zero. The refrigerant inside the outdoor coil absorbs this low-level heat, boils into a gas, and is compressed by the compressor, which dramatically raises its temperature before releasing the heat indoors. However, because moisture in the freezing air condenses and turns into frost on the cold outdoor coil, the heat pump periodically initiates an automatic defrost cycle. During this cycle, the system temporarily reverses its operation to melt the ice, while auxiliary electric resistance heating strips engage to prevent the indoor air from turning cold. You can verify proper operation when the system maintains indoor warmth and automatically clears frost.
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