Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I've used spreadsheets do do this in the past, but I can't seem to find one right now. But the calculation isn't to difficult to do by hand. In AirflowNetwork and similar models, the flow between two nodes in the network is computed as
mass_flow = f(pressure_difference)
where the function f is determined by the input fields of the particular object. Given enough information (and probably an assumption or two), you can calculate the needed fields. AFN:MultiZone:Surface:Crack uses a power law formulation [1] and a specified reference state, so pick a reference temperature and pressure difference that makes sense and solve
mass_flow = C_Q * pressure_difference^n
for C_Q. For this one, the easy thing to do would be to use n=0.65. That's basically it, but do be careful about the crack factor that comes from the surface that the crack is attached to. For other elements, the procedure is relatively similar, but in other cases it may be harder to make sure that the right reference state is used. We've been adding Python scripts to automate things like this, if that's something you'd be interested in seeing please put in a feature request on the EnergyPlus GitHub repo.
[1] [https://bigladdersoftware.com/epx/docs/9-3/input-output-reference/group-airflow-network.html#airflownetworkmultizonesurfacecrack](https://bigladdersoftware.com/epx/docs/9-3/input-output-reference/group-airflow-network.html#airflownetworkmultizonesurfacecrack)
