A tech here at Appliantology was called out to work on a refrigerator that wasn't defrosting. However, while he was there, something else caught his eye:

That's not styrofoam -- that's a thick layer of frost completely covering the suction line of the compressor. 

What would cause a symptom like this? Why isn't this refrigerator keeping the cold in the places where it's supposed to be? Do we have a sealed system issue on top of our defrost problem?

To answer these questions, let's do a quick review of how a refrigerator's sealed system works.

This is a simple representation of what's going on in a refrigerator's sealed system. I've covered how exactly the refrigerant cycle works in detail, but as a brief refresher:

The compressor pumps the refrigerant through the system. As you can see from the different colored arrows, as the refrigerant moves through the sealed system, it changes state from liquid to gas and vice versa due to pressure changes and the absorption/shedding of heat energy.

While in the condenser coils, you see the red arrows change to blue arrows. That's because the vapor refrigerant sheds heat in the condenser, turning in to a liquid as it does so. After passing to the low pressure side of the system (the two halves are divided by an expansion valve here, but we more typically see a capillary tube -- same function, different methods), this liquid refrigerant absorbs heat energy in the evaporator coils, which makes it turn to gas again before returning to the compressor.

That last part in the evaporator is our focus today. Imagine what would happen if liquid refrigerant got all the way through the evaporator coil without boiling. What happens to it now?

Well, as you can see, after the evaporator, it goes through the suction line toward the compressor. And if it didn't get the chance to boil in the evaporator, then it will definitely boil in the suction line. Boiling means absorbing heat, and when you absorb quickly like that, you create frost.

So that explains our symptom. A frosted suction line always means that there is liquid refrigerant leaving the evaporator.

Sealed systems are engineered so that this doesn't happen. But if a system were to be charged with too much refrigerant, then you get a situation where not all of the refrigerant is able to boil before it gets pushed out of the evaporator. 

So is that our answer? Do we have a refrigerant overcharge in addition to our defrost problem?

Not so fast. While an overcharge can cause a frosted suction line, it's not the only thing that can. Too much refrigerant is one thing, but what if there's a normal amount of refrigerant, but something is preventing it from absorbing heat from the environment? Something like... a heavily frosted evaporator coil?

As people who live in the Arctic have known for many years, ice is a great insulator Too much frost on those evaporator coils, and you won't get enough heat transfer to boil all the refrigerant, no matter how much of it you pump through. But just like our other example, it will happily absorb heat through the suction line and make more frost there.

So that's the answer to our problem -- as usual, the two seemingly disconnected symptoms have a common cause.

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