Which of the following could cause low superheat or liquid floodback to the compressor?

• A. A drastically oversized system
• B. Foreign material lodged under the expansion valve
• C. Dirty evaporator coil
• D. All of the above

Answer: (D)

Low superheat means the refrigerant isn’t fully vaporizing in the evaporator, so liquid refrigerant can reach the compressor—this is called floodback. Any condition that prevents complete vaporization or causes too much liquid to enter the evaporator can produce that floodback.

An oversized system can cause this because it may not load with enough heat, so the evaporator isn’t driven to produce sufficient vapor. If foreign material is lodged under the expansion valve, the metering can be disturbed, allowing too much liquid refrigerant into the evaporator. A dirty evaporator coil impedes heat transfer, meaning the refrigerant doesn’t absorb enough heat to vaporize fully. In each case, liquid can slip through to the compressor, lowering the observed superheat. Therefore, all of these scenarios can contribute to low superheat or liquid floodback.