CSLB C-20 HVAC ContractorCooling and RefrigerationHard
A technician observes that a TXV-equipped system's suction pressure and superheat readings oscillate repeatedly, cycling between high and low values every few seconds instead of stabilizing. This behavior, known as valve 'hunting,' is most commonly caused by:
- AAn oversized TXV or an improperly located/insulated remote sensing bulb
- BLow refrigerant charge with reduced subcooling
- CNon-condensable gases trapped in the system
- DA restricted liquid line filter drier
Show answer & explanationAnswer & explanation
Correct answer: A. An oversized TXV or an improperly located/insulated remote sensing bulb
Valve hunting occurs when the TXV overreacts to temperature changes at the sensing bulb, repeatedly overfeeding and underfeeding the evaporator. Common causes are an oversized valve for the load, or a bulb that is poorly located, poorly insulated, or improperly clamped, causing it to respond to ambient conditions rather than actual suction line temperature.
Why the other options are wrong
- B. Low charge produces consistently low subcooling and high superheat, not cyclical hunting.
- C. Non-condensables cause elevated head pressure, not superheat oscillation.
- D. A restricted drier causes a pressure drop and low subcooling, not oscillating superheat.
TXV Hunting
A condition where a TXV repeatedly overfeeds and underfeeds the evaporator, causing superheat and suction pressure to oscillate, usually due to an oversized valve or poor bulb installation.
- Oversized TXV reacts too aggressively to small temperature changes
- Bulb must be properly clamped and insulated on the suction line for accurate sensing
- Correct bulb placement and properly sized valve prevent hunting
Memory trick: An oversized valve overreacts like a hunter chasing a moving target.