Height difference (ΔHNPL) in ventilation stack in EnergyPlus

Hi!

The formula EnergyPlus uses for stack effect in ventilation is as follows: Qs=Cd * Opening Area * Schedule * SQRT(2gDH*(|(Tzone-Todb)|/Tzone))

The DH (referred as as "ΔHNPL" in the Inputoutput reference ) is defined as the height difference between the midpoint of an opening and the neutral pressure level. But EnergyPlus has considered the value of zero as default.

Doesn't this value of zero, makes the whole formula zero and consequently Qs=0?

Thanks a lot in advance!

Yes, the default is zero for this input which means the ventilation due to stack effect by default is also zero. It is better to have no impact than an erroneous impact. This is a parameter that is complicated to determine and EnergyPlus does not try to do this automatically so instead the InputOutputReference says:

ΔHNPL = Height from midpoint of lower opening to the neutral pressure level [m]. Estimation of this value is difficult; refer to Chapter 16 of the 2009 ASHRAE Handbook of Fundamentals for guidance.

In Chapter 16 of the ASHRAE Handbook of Fundamentals (2009) is a section on Neutral Pressure Level that should be reviewed as well as Thermal Draft Coefficient that follows it. These same sections also appear int eh 2013 version of the handbook.