VAVRSVirginia Association of Volunteer Rescue Squads, Inc.
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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 62.1 every hour of the year and the annual average outside airflow is 260% larger than Standard 62.1 rates including multiple space inefficiencies.
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.
It is possible with VAVR systems to locate many boxes in corridors or non-critical spaces outside the zones they serve, so that maintenance or repairs can be done without disrupting the occupied space; but no such option exists for ACBs.
Rigidly secured and piped chilled beams are much more expensive to relocate than VAVR diffusers, which do not require seismic bracing and are generally connected with flex duct.
This credit is generally achievable with a VAVR system since it requires that all supply air be filtered, but not with an ACB system because the secondary air induced through chilled beams is unfiltered.
As discussed earlier, the ACB+DOAS design has a higher minimum zone cooling output than the VAVR design and is likely to be less comfortable.