Modeling natural ventilation

Energy modeling for LEED submission was done on a recent project in which natural ventilation and a radiant floor cooling system were proposed for the dining area of a food court in a local university in Southern California.

Erin McConahey, PE, Arup, Los Angeles

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Energy modeling for U.S. Green Building Council LEED submission was done on a recent project in which natural ventilation and a radiant floor cooling system were proposed for the dining area of a food court in a local university in Southern California. EQuest was used because it was the most appropriate software at the time, but its developers did not envision such a building, so “workarounds” had to be created to mimic the actual energy use of the building.

Because the design incorporated a mixed-mode approach, and none of the currently available engines allows for multiple systems serving the same space, it was necessary to create the radiant systems as serving “dummy” zones with simulated cooling coils served by fans with 0.0001 kW/cfm energy (because zeroing out fan energy in EQuest can cause instability). Furthermore, it was necessary to perform a dynamic heat transfer analysis in an alternate program to represent a schedule of operation to represent when the natural ventilation was active (in the typical meteorological year file) and when the air conditioning units had to be scheduled to be on for the primary zone.

As shown in Figure 3, there is no way graphically to represent the systems based on the available components in the Schematic Design Wizard. However, with the detailed modules of EQuest and a substantial amount of review of the *.sim files, it was possible to get a reasonably good representation of the energy performance. This was an improvement on the manipulation of baseboard heating modules to simulate radiant floor heating and allowed the computation of cooling energy. While EQuest does have a natural ventilation capability, the complexity of changeover systems for mixed mode approaches did require the heavier manual manipulation of files to represent all of the energy components adequately and meet the expectations of the LEED reviewer panel.