OpenStudio merged surfaces inside a thermal zone to internalmass

In OpenStudio when I assign multiple spaces into one thermal zone, in the exported IDF, the partition walls between these spaces gets merged into "InternalMass" in EnergyPlus. However, looks like the "SurfaceArea" of the InternalMass is only one wall area. Since EnergyPlus asks for the "total surface area exposed to the zone air" should that be two times the original wall area since both sides of the partition wall are exposed to the zone air?

Good question. This sounded familiar to me, and looking back we did have an old issue filed about it https://github.com/NREL/OpenStudio/is...

Currently we do only model half the exposed area we should for the internal mass, but if we just double the area then we will have too much mass. Sounds like the best solution would be to double the area while at the same time altering the construction to cut down the mass. Maybe can just cut thickness of materials in half. A more complex solution would split the construction down the middle (or at least conductive layer) into two constructions.

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David Goldwasser

answered 2015-09-01 09:08:12 -0500, updated 2015-09-01 09:09:54 -0500

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There is a related issue. This goes into more detail about other improvements to internal mass. Specifically it talks about combining all the internal mass objects within a zone that use the same construction into a single internal mass object to speed up simulation; but no single internal mass object should be bigger than half of the area of the heat transfer surfaces in a zone.

David Goldwasser (2015-09-01 09:20:06 -0500) edit

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OpenStudio merged surfaces inside a thermal zone to internalmass

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OpenStudio merged surfaces inside a thermal zone to internalmass edit

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OpenStudio merged surfaces inside a thermal zone to internalmass