What condition can cause frost to form on the top of a coil while the bottom remains dry?

Frost formation on the top of a coil while the bottom remains dry typically indicates a shortage of refrigerant in the system. When there is insufficient refrigerant, the coil does not receive the necessary amount to vaporize properly throughout its entire length. This can lead to uneven cooling, where the top portion of the coil may become cold enough to cause moisture in the air to condense and freeze, while the lower portion remains warmer and dry due to inadequate refrigerant flow.

In a well-functioning system operating under normal conditions, the refrigerant should distribute evenly, leading to consistent temperatures across the coil. Excess refrigerant could potentially cause flooding and performance issues, while a dirty coil would typically lead to frost across its entire surface rather than localized frost. Understanding these conditions helps in diagnosing refrigerant-related issues effectively.