NatVent Window Area Bug

In trying to model an operable window, I came across a strange bug. When I compared the HB readMe to the IDF written, the window areas did not correspond. Furthermore, when I increased the HB window area, the IDF window area decreased. See screenshots below and GH file attached.

HB readMe says that the operable window area is 5.4 m2:

…but this writes an IDF with only 0.5 m2 operable area:

When the operable area is increased to 10.8,

HB writes an IDF with only 0.26 m2 operable area.

I’d love to know if anyone has any idea what’s going on… (506 KB)

Hi Burin,

Chris will get back to you on this soon. It looks like a bug to me. The report is printing a different value than the effective ventilation area. I added an issue on github (

As always thank you so much for double checking and reporting the issues! Highly appreciated.


PS: @Chris- I have no idea why images that upload look like this but I hope it shows where the issue is!


Thanks for finding this bug. I think you are setting the record for the most number of bugs found in the shortest amount of time.

It seems that I originally over-complicated the calculation of effective area by looking at my old textbooks. In this case of the natural ventilation here that is mostly meant for single-zones, I realized that the effective area and opening are are one in the same.

I have since fixed the bug and you can find the new component in the attached file and on the github.

I should warn that, when you use wind ventilation as you do in the attached GH file, there is an assumption that there is unobstructed cross ventilation from one side of the zone to the other. Normally, for closed test boxes with windows on only one side like that in the attached file, I would set the windDischargeCoefficient to 0 and only look at natural ventilation happening through buoyancy driven flow.

-Chris (506 KB)

Much appreciated. I wouldn’t be reporting so many bugs if you guys weren’t so fast at fixing them! :slight_smile: