What can cause frost accumulation on the outdoor coil of a heat pump in heating mode when the ambient temperature is 45°F?

Frost accumulation on the outdoor coil of a heat pump in heating mode, even at an ambient temperature of 45°F, occurs when the outdoor coil is colder than the surrounding air. In heating mode, the heat pump extracts heat from the outdoor air to warm the indoor space. This process cools the outdoor coil to below the dew point of the air. When the temperature of the coil drops below this point, moisture in the air can condense and freeze on the coil, leading to frost accumulation.

In this scenario, even though 45°F is relatively mild, the heat extraction process can lead to the outdoor coil being significantly colder, causing frost to form under the right humidity conditions. Understanding the relationship between the coil temperature and ambient air temperature is crucial to grasp why this condition arises. This is particularly vital for diagnosing performance issues and maintaining the efficiency of heat pumps in various operating conditions.