Wednesday, August 30, 2006

eQuest Displacement Ventilation

Topic: What adjustments to the baseline model are required to simulate a displacement ventilation system?

Energy Design Resources has a reference for doing underfloor and displacement ventilation systems with eQuest. While useful since eQuest (version 3.61 as of this writing) does not yet natively support displacement ventilation, the information is evidently based on an earlier version of eQuest prior to implementation of the 'lighting heat to adjacent space and return air' features shown in the following dialogue (which may be found by doing a 'detailed edit' on spaces in the component tree):

(click on the image to see a larger version)

Until eQuest is released with specific displacement ventilation modeling capability, the following workaround may be used to approximate a displacement ventilation simulation solution:
  • Assign a higher fraction of the lighting heat load to the return air for displacement ventilation over the baseline case.
    • If you've accepted the default of 100% lighting heat load to the occupied space for the baseline case, then for example a 50-50 split may be assigned for the displacement ventilation scenario.
  • If there is significant occupant load per square foot, then assign some additional fraction of the lighting load to the return air to account for it, since there is not an explicit option for doing so.
    • Continuing the example above, in school classrooms perhaps a split assigning 70% of the lighting load to the return air is a better approximation for the displacement ventilation scenario.
  • Also adjust supply air delivery temperatures, supply air volumes, and drybulb economizer high limit per the proposed displacement ventilation design; consult with the mechanical engineer.
  • Typically the supply air and drybulb high limit temperatures will be somewhat higher and the overall supply air volume somewhat lower than a baseline overhead distribution system.
If the project is fortunate enough to be located in a temperate area that does not require mechanical cooling in the spring and fall and is unoccupied during the summer (e.g. a school in western Washington State), then don't expect much in the way of reduced energy consumption. Displacement ventilation can only provide estimable savings by avoiding mechanical cooling.

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