If you see frost or rime ice in a freezer, then you know you're really looking for water vapor that's getting into the compartment. How does water vapor get into a refrigerator? It comes in with the outside air. In most cases when you see frost in a freezer, you're looking for an air leak: bad door gaskets or doors not closing all the way. This video shows an extreme example of rime ice all over the contents inside a freezer:
Sometimes, you'll see both ice and frost appearing together in a freezer which can make diagnosis tricky. In this video, I walk you through an example of such a case and I explain the failure sequence:
A special (but common) case for diagnosing frost in a freezer is when you see frost accumulated on the evaporator coil or back wall inside the freezer that covers the evaporator coil. This indicates a defrost system failure (defrost terminator stuck open, burned out defrost heater, bad defrost timer (on older units) or adaptive defrost control (ADC) board).
The reason rime ice forms on the evaporator coil in the first place is because the coil operates at a temperature of -20F. At that temperature, water vapor that contacts the coil will condense and freeze directly into a solid, forming rime ice. Every few hours the defrost system should kick in and melt that ice, because if it's allowed to accumulate it will eventually act as an insulator, preventing the air from contacting the evaporator coils and getting cold. The resulting problem would first be seen as a warm refrigerator compartment and, if allowed to continue, eventually the freezer will also get warmer than normal (normal = 0F). Rime ice accumulated on the inside of the back wall in the freezer will often be seen at this point.
This melted rime ice has a special name: condensate. (Not to be confused with condensation, although the words are similar, they arise from two different causes.) Condensate refers to the water that gets melted off the evaporator coil in the freezer compartment during the defrost cycle. This condensate drips onto the condensate drip trough below the evaporator coil and drains out the condensate drain-- a hole in the condensate drip trough-- through a tube to the drain pan placed down by the compressor where it eventually evaporates due to the combined action of the compressor heat and condenser fan motor.
This video shows a freezer with extreme rime ice buildup on the back wall inside the freezer due to a defrost system failure:
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