A normally open control valve is open at what air pressure?

A normally open control valve is designed to remain open when there is no air pressure applied to it. In this context, when the air pressure is at 0 PSI, the valve stays open, allowing flow to occur freely. When pressure is introduced, such as in the options presented, the valve will start to close. This means that at 0 PSI, the valve fully retains its open position, not requiring any pressure to maintain that state.

The other options involve positive pressure levels where the valve will begin to close as pressure increases. Therefore, the indication that 0 PSI is when the valve is fully open aligns with the foundational principles of normally open valve operation in HVAC systems.