Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Unable to load the ChaptGPT conversation. Maybe consider posting a print screen image? Curious to read its take on this.
My understanding is that the SurfaceProperty:ExteriorNaturalVentedCavity class is inspired (or derived) from the SolarCollector:UnglazedTranspired class (very similar inputs, terminology, etc.). Based on Line 745, EnergyPlus resets the user-defined height scale (or delta-NPL) based on the tilt of the surface. If the surface is near horizontal, EnergyPlus picks either the reset delta-NPL or the cavity thickness (i.e. the largest of the 2 values). In fact, it looks like the gap thickness must be greater than zero (Line 702). So I'd say it's quite safe to provide both height and gap thickness inputs, even if the ventilated facade is vertical
| | 2 | No.2 Revision |
Unable to load the ChaptGPT conversation. Maybe consider posting a print screen image? Curious to read its take on this.
My understanding is that the SurfaceProperty:ExteriorNaturalVentedCavity class is inspired (or derived) from the SolarCollector:UnglazedTranspired class (very similar inputs, terminology, etc.). Based on Line 745, EnergyPlus resets the user-defined height scale (or delta-NPL) delta-Hnpl) based on the tilt of the surface. If the surface is near horizontal, EnergyPlus picks either (i) the reset delta-NPL delta-Hnpl or (ii) the cavity thickness (i.e. the largest of the 2 values). In fact, it looks like the gap thickness must be greater than zero (Line 702). So I'd say it's quite safe one is required to provide both height and gap thickness inputs, even if the ventilated facade is verticalvertical.
| | 3 | No.3 Revision |
Unable to load the ChaptGPT conversation. Maybe consider posting a print screen image? Curious to read its take on this.this. EDIT: Thanks for adding it.
My understanding is that the SurfaceProperty:ExteriorNaturalVentedCavity class is inspired (or derived) from the SolarCollector:UnglazedTranspired class (very similar inputs, terminology, etc.). Based on Line 745, EnergyPlus resets the user-defined height scale (or delta-Hnpl) delta-Hnpl) based on the tilt of the surface. If the surface is near horizontal, ~horizontal, EnergyPlus picks either (i) the reset delta-Hnpl delta-Hnpl or (ii) the cavity thickness cavity thickness (i.e. the largest of the 2 values). In fact, the gap thickness cavity thickness must be greater than zero (Line 702). So one is required to provide both height scale and gap thickness cavity thickness inputs, even if the ventilated facade is vertical.
EDIT (in response to follow-up comments): As stated in the IO Reference, EnergyPlus does extract surface area from the linked BuildingSurface:Detailed when it comes to baffle/gap geometry. Aside from that, there's no way for EnergyPlus to guess the actual baffle details from BuildingSurface:Detailed geometry alone. So yes: one must provide the height scale (not the actual height of the BuildingSurface:Detailed) and the effective thickness of the cavity (not the thickness of any construction Material layer). The Engineering Reference section may help to better understand these differences. BTW, a recent UMH post on the question.
So in summary, EnergyPlus actually does need both the real height scale or the real cavity thickness for its calculations. It's just that it can't autoextract these from surface geometry or material layers. And as stated in the IO Reference excerpt you originally quoted, the cavity thickness value "is only used when the collector is near horizontal to determine a length scale in the vertical direction for buoyancy calculations".
I haven't checked, but I suspect the cavity thickness is likely used to output some related variables, like:
This requires cavity volume (i.e. cavity thickness x height of the BuildingSurface:Detailed?). You could test this out by running 2 simulations, where you double the (vertical) cavity thickness for the 2nd simulation. I suspect the reported ACH would change, but not necessarily these other reported variables?
Not sure, but may help.
I think ChaptGPT ended up answering another question: how to determine an equivalent thickness of an insulating material (Construction Material), as a proxy for a vented/ventilated air gap. Not a bad answer BTW, but beside the point when it comes to EnergyPlus' SurfaceProperty:ExteriorNaturalVentedCavity. A bit of further question refinement may have yielded better results. Next time, maybe.
