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# AirflowNetwork model error in EnergyPlus

I am encountering the following warning message while modelling AirflowNetwork in EnergyPlus:

** Warning ** GetAirflowNetworkInput: SurfaceAverageCalculation is entered for field = Wind Pressure Coefficient Type, but no roof surface is defined using an AirflowNetwork:MultiZone:Surface object.
** ~~~ ** Reconsider if this is your modeling intent. Simulation continues.
** Warning ** GetAirflowNetworkInput: SurfaceAverageCalculation is entered for field = Wind Pressure Coefficient Type.
** ~~~ ** The AirflowNetwork model provides wind pressure coefficients for 4 vertical exterior orientations and 1 horizontal roof.
** ~~~ ** There are only 0 exterior surface orientations defined in this input file using AirflowNetwork:MultiZone:Surface objects.
** ~~~ ** Reconsider if this is your modeling intent. Simulation continues.


I have clearly defined all the exterior surfaces (Walls and Roof). I can however, see the wind pressure coefficients that are generated in the output. Could you please shed some light on it.

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If you have roof surfaces, then the problem is likely that E+ is not identifying them as roof surfaces. The coefficient curves are generated and then the surfaces are connected to the curves (via generated external node objects). The code that does all that is in AirflowNetworkBalanceManager.cc (look at the CalcWindPressureCoeffs routine). If you have tilted roof surfaces then those surfaces may be identified as walls rather than roofs. The "SurfaceAverageCalculation" approach uses a flat roof correlation, so that restriction is pretty much required.

Have you tried using "Input" instead? That will require defining the external nodes yourself, so it is more work, but would also help determine if there is something else going wrong.

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hi Jason,

Thank you for your suggestion. I have checked the AirflowNetworkBalanceManager. It appears from the calculation sequence that the surfaces and nodes are first identified and then pressure coefficients are calculated. However, I can also see pressure coefficients for 4 surfaces in the .eio output file. The roof is a flat roof and so the value might be 0. I have varied the geometry parameters and definitions but to no avail. I want to do a parametric simulation with varying side ratio and orientation, so it might be cumbersome to use "Input".

( 2017-09-20 17:30:21 -0500 )edit

There is no problem when I tried using "Input". However, the problem still persists with "SurfaceAverageCalculation".

( 2017-09-21 17:31:18 -0500 )edit

Horizontal surfaces should get identified as roofs, so If you're sure that the roof surfaces are horizontal then you might have found a bug. You can submit it over here. Make sure to attach an input file that generates the warning. Definitely stick with "Input" for the time being, that's the safest option until the next release (8.8 will be out soon, it as some bug fixes in the routines that do these calculations).

( 2017-09-23 16:58:47 -0500 )edit

Thank you for your suggestion. I will submit it there with a single zone simple input file.

( 2017-09-24 04:41:16 -0500 )edit

The file you posted over on GitHub doesn't appear to have sufficient AFN surface objects to avoid having this warning issued. The code expects more surfaces than you are providing, so it is complaining. That doesn't mean that the simulation won't work, but you should really think about whether this is what you want or not.

( 2017-09-27 14:38:15 -0500 )edit

In spite of the warning it appears that the program is calculating the right wind pressure coefficients. Output results AFN Zone Infiltration Mass kg and AFN Node Wind Pressure Pa show different values when the AirflowNetwork:SimulatonControl input parameters Azimuth Angle of Long Axis of Building and Ratio of Building Width Along Short Axis to Wind Along Long Axis are changed. I am still waiting to hear from the help desk and other experts though, for now this seems to be working. EIO output file also shows the actual CP values generated for a set of these two specific input parameters.

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