Net ZEB Design Processes and Tools - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 198

(windows, walls, and doors), though some tools (e.g., ESP-r) allow the use
of artificial or “fictitious” surfaces to represent openings. Most BPS tools
assume that the air within the zones is perfectly mixed, though more
complex models consider several vertically stacked air masses for vertical
stratification or use multidimensional CFD.
A building should be discretized into thermal zones such that any space
with significantly different heat gains, solar exposure, operating conditions
(e.g., temperature), and occupancy patterns are unique. Furthermore, the
zonal configuration should reflect the HVAC control zones (primarily for
detailed design) (Department of Energy, 2013c). O'Brien, Athienitis, and
Kesik (2011b) showed that the tendency to model buildings with few zones
in early design stages can under-predict energy use and overpredict thermal
comfort. They modeled a two-story house with one, two, three, and five
zones and plotted the heating and cooling energy use for the different
south-facing window sizes (see Figure 4.13 ). The results in Figure 4.14
clearly show that modeling the house as a single zone not only reduces the
predicted energy use but also increases the optimal window-to-wall ratio.
This is because the single zone configuration assumes that all solar gains in
the southern part of the house are instantaneously distributed to the entire
house (perfectly mixed air assumption). In reality, the solar gains would
likely only heat up the southern part of the house and could cause the need
for simultaneous heating (northern part of the house) and cooling (direct
gain zone).