Other Downsides of ACB versus VAVR Water Leaks. 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. 450 Pumps/Misc. Towers Fans Cooling Heating 400 Site TDV Energy kBtu/ft2·Year compliance with the California Title 24 energy code and to determine LEED energy credits. The Title 24 modeling rules state the baseline model shall have the same outdoor air ventilation rate as the proposed model. EnergyPro interprets this to mean the same percentage of outside air. EnergyPro also considers ACB+DOAS to be a single zone system and thus compares it to constant volume, 100% outside air, packaged single zone units. On the other hand, EnergyPro compares the VAVR design to a packaged VAV reheat system with the same minimum ventilation rate. 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. However, it also shows the ACB+DOAS baseline is three times higher than the VAVR baseline! Fortunately, the California Energy Commission is currently revising the Title 24 modeling rules so that the HVAC design and ventilation rate in the baseline model are fixed regardless of the proposed HVAC design or proposed ventilation rate. 350 300 25 200 150 100 50 0 ACB + DOAS ACB + DOAS Baseline VAVR VAVR Baseline Figure 14: EnergyPro HVAC energy simulation results. Maintenance. 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. Lighting. Lighting quality and efficiency may be compromised due to restrictions in fixture location imposed by chilled beams. www.info.hotims.com/44632-51 May 2013 ASHRAE Journal 29