Why should an evaporator not be placed above a door in a walk-in cooler or freezer?

Placing an evaporator above a door in a walk-in cooler or freezer is problematic primarily because it would draw in warm, humid air whenever the door is opened. This influx of warm air increases the load on the evaporator, causing it to work harder to remove the added heat and humidity from the space. This not only leads to inefficiency but also results in rapid frost buildup on the evaporator coils due to the condensation formed from the humid air. Excessive frost accumulation can lead to decreased cooling efficiency and can even cause the evaporator to fail if it becomes excessively blocked.

The other choices touch on important factors, but they do not directly address the core issue of humidity influence and operational efficiency associated with drawing in warm air through the door. Understanding this aspect emphasizes the importance of strategic placement of evaporators to maintain the efficiency and functionality of refrigeration systems.