Which of the following is a potential cause of overfeeding the TEV?

• A. Oversized TEV
• B. Low superheat adjustment
• C. TEV sensing bulb not making good thermal contact
• D. TEV not receiving power

Answer: (A)

Thermostatic expansion valves meter refrigerant to the evaporator based on the evaporator load and the suction-line temperature to keep the evaporator’s superheat at the set point. The valve’s capacity to pass refrigerant is limited by its size and design, so the amount of flow is a balance between how open the valve is and how much the evaporator can accept.

If the valve is oversized for the system, it has more flow capacity than the evaporator needs. Even with a nearly closed position, an oversized valve can still pass more refrigerant than the evaporator can vaporize. That extra liquid refrigerant entering the evaporator leads to overfeeding, where the evaporator gets flooded with liquid and the evaporator output isn’t properly vaporizing before returning to the compressor.

Other factors can disrupt operation, such as the sensing bulb not making good thermal contact, which can cause incorrect readings and unstable flow, or a setting that tries to control to a very low superheat, but these aren’t as direct and reliable a cause of persistent overfeeding as using a valve that’s too large for the system. The valve isn’t powered, so it doesn’t depend on any external power source.

So, the best answer is that an oversized TEV tends to overfeed the evaporator because its capacity exceeds what the system load requires.