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# Height Scale for buoyancy-driven (EnergyPlus)

Thank you for your help in advance. I would like to know how to determine the Height Scale for buoyancy-driven ventilation which is part of the class "SurfaceProperty:ExteriorNaturalVentedCavity" in EnergyPlus. I have read somewhere that it is usually the half of the total height of the facade, but I am not sure about this answer. I do reiterate my gratitude.

UPDATE

We are currently modeling an apartment on the fourth floor of a building, with the floor located 13 meters above ground level. While we're not sure of the regulations in Canada, in Spain, any cavity in the façade must be interrupted at each floor level to prevent the spread of fire in the event of an emergency. Accordingly, the cavity in our model is interrupted on every floor, approximately every 3 meters, as shown in the attached drawing (image adapted from the one published on this website: https://acortar.link/lNaOAf).

We hope you don't mind if we ask for further clarification. In our case, a four-floor apartment with the cavity "insulated" as described above, should the Height Scale for Buoyancy-Driven Ventilation be calculated as half the floor height (i.e., 1.35m)? Or should it be considered as 14.35 meters (13m + 1.35m) to account for the change in wind speed with altitude?

Attached drawing: https://ibb.co/NLzs7QG

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In theory, for a vertical façade cavity under strictly buoyancy-driven flow (no wind), and with equally-sized and equally-spaced openings at the top and bottom of the cavity, the neutral pressure level (or NPL) should be at mid-height of the vented cavity. If cavity openings are neither equally-spaced nor equally-sized, then the NPL (under strictly buoyancy-driven ventilation) would either shift up or down from mid-height. I suggest the following reference (2nd to 3rd page).

In EnergyPlus, I think there are a few built-in simplifications (see Engineering Reference). From what I gather, one assumption is that top and bottom openings are equally-sized and positioned, and are coplanar with the cavity façade (or baffle). So the NPL is assumed to be mid-height of the cavity, and the height scale is "defined as the height from the midpoint of the lower opening to the neutral pressure level". So not half the total height of the cavity, but close.

I'm assuming (and may be quite wrong) that the lower and upper openings are assumed to be really near the bottom and top of the baffle, yet the height scale sets in part the geometry of the openings (as "increasing the value [of the height scale] will increase the ventilation rate due to buoyancy"). Hope this helps.

EDIT: See original NREL report.

EDIT 2

In response to your follow-up question (1.35m vs 14.35m) under "Update": Based on your description, the height scale should be approximately (or slightly less?) than 1.35m for a 2.70m compartmentalized cavity.

EnergyPlus will take care of the change in wind speed based on the 3D position of the referenced BuildingSurface:Detailed.

Best of luck!

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