The VAVR design uses 30% more sheet metal than the ACB+DOAS design, but the ACB+DOAS design uses 30 times more chilled water piping and five times more hot water piping.
Many other potential costs are not included in these figures that could make the ACB+DOAS design even more expensive relative to VAVR.
The VAVR system was clearly better on both energy efficiency and HVAC costs compared to the ACB+DOAS system, but what about the other supposed advantages of ACB+DOAS?
On the same floor, the VAVR design has one 54 x 16 in.
To mitigate the fan energy penalty of a constant volume system relative to a VAV system, an ACB+DOAS system must use lower duct velocities compared to a VAVR system.
An ACB+DOAS design might have a floor-to-floor advantage over VAVR with different design choices.
Proponents of ACB+DOAS argue it provides better indoor air quality than VAVR because it uses higher design ventilation rates.
Figure 13 shows the actual outside airflow in the VAVR design met or exceeded the outside airflow required by Standard 62.
VAVR, of course, is not vulnerable to this potential IAQ risk.
There is one area the ACB+DOAS design significantly outperformed the VAVR design: LEED energy points.
Figure 14 shows the ACB+DOAS design uses 25% less HVAC energy than its EnergyPro-generated baseline while the VAVR Design only uses 15% less HVAC energy than its baseline.
In addition to the condensation risk, ACB systems also have a much higher risk of chilled and hot water leaks than VAVR systems because they have so much more piping, particularly over tenant spaces.