What should the branch pressure be leaving the thermostat to control a N/O VAV box when the surrounding temperature and desired set point are both 70 degrees Fahrenheit?

The branch pressure leaving the thermostat to control a normally open (N/O) VAV (Variable Air Volume) box should ideally be 0 psig in this scenario. When the surrounding temperature and the desired set point are both at 70 degrees Fahrenheit, it indicates that the space is already at the desired temperature. Therefore, there is no need for additional pressure to push conditioned air into the VAV box, as the system's primary function is to maintain the desired temperature.

Using 0 psig allows the VAV box to remain in an idle state until there is a demand for cooling or heating when the space temperature deviates from the set point. This setup helps the system to operate efficiently, preventing unnecessary energy use by avoiding the circulation of conditioned air when it's not required.

In contrast, having a non-zero pressure, such as 5 psig, 10 psig, or 15 psig, suggests that there is still an active flow of air being pushed into the VAV box, which is unnecessary and can lead to increased energy consumption or discomfort in the controlled space.