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The opening pressure in a Thermal Expansion Valve (TEV) is determined by the bulb pressure. The bulb is connected to the sensing element of the TEV, and as the temperature in the evaporator rises, the refrigerant in the bulb expands. This increase in bulb pressure is what ultimately pushes against the spring pressure to open the valve.

When the bulb pressure exceeds the spring pressure, the TEV opens, allowing refrigerant to flow into the evaporator and maintaining the system's efficiency and temperature control. The proper functioning of the TEV is crucial for both cooling and heating cycles, as it regulates refrigerant flow based on evaporator temperatures.

While the other pressures—evaporator pressure and spring pressure—play important roles in the overall operation of the TEV, it is the bulb pressure that directly triggers the opening of the valve, making it the key factor in determining when the TEV opens. A combination of bulb and spring pressures influences how the valve operates but is not the defining pressure for the opening action.