What pressure is sensed by the TXV's sensing bulb?

• A. Evaporator pressure
• B. Suction bulb pressure
• C. Condenser pressure
• D. Discharge pressure

Answer: (B)

The sensing bulb on a TXV sits on the suction line near the evaporator and reads the low-side, evaporator pressure. Inside the bulb is a small amount of refrigerant whose pressure changes with temperature; as the evaporator temperature (and thus the suction/evaporator pressure) rises, the bulb pressure increases and acts on the valve diaphragm to reduce flow. When the evaporator temperature falls, bulb pressure drops and the valve opens more to feed more refrigerant. So the pressure being sensed is the suction/evaporator pressure on the low side, not condenser or discharge pressures.