Our project is a 4 story 120,000 sq. ft. laboratory/classroom building with fossil fuel heat. Table G3.1.1A indicates that the baseline system should be packaged VAV with reheat (System 5). ASHRAE 90.1-2004 G3.1.1(c) indicates that if System 5 is our baseline, we should use separate single zone systems conforming to System 3 for any zones with special pressurization relationships or cross contamination requirements. Since the labs in our building have special pressurization relationships and cross contamination requirements, we are modeling the lab spaces with System 3 as the baseline per G3.1.1(c). The non lab spaces (without pressure or contamination requirements) will utilize System 5 as the baseline. It is not stated in ASHRAE 90.1-2004 G3.1.1 whether System 3 should be modeled with or without reheat. When we model the lab spaces with System 3 without reheat, we cannot maintain temperature and humidity at conditions acceptable for the experiments and for the occupants and the unmet load hours will not comply with G3.1.2.2. Modeling the lab spaces with reheat maintains the temperature and humidity compliance ranges of G3.1.2.2. These results were expected since these systems are near 100% outside air, and System 3 without reheat would not work for most labs in real life. In addition, ASHRAE 90.1-2004 6.5.2.1 (b) allows reheat for zones with special pressurization relationships and cross contamination requirements if VAV systems are impractical. While VAV systems are practical for this lab and are being used in the actual design, the minimum turn-down ratio required by G3.1.3.13 of 0.4 cfm/ft2 does not fall within the accepted current guidelines for Air Change Per Hour rates of OSHA (4-12), NFPA (min of 4), ASHRAE Applications (min of 3 for BSL-1 and min of 4 for BSL-2 spaces), and others in the same range of air flow rates. A typical lab space with a 9.5 feet ceiling height results in 0.63 cfm/ft2 at 4 ACHR, which is much higher than the required limit. ASHRAE 90.1-2004 G3.1.1(c) is specifically requiring us to use a constant volume system in the baseline but does not indicate any reheat limitations. Can System 3 have reheat for a lab baseline system? If not, please indicate the proper baseline system that would actually work for a laboratory space in real life, since System 3 without reheat or System 5 with a turndown ratio of 0.4 cfm/ft2 do not seem like acceptable baselines.
The project is requesting clarification for the system type selection for the specific project. If the proposed system is a VAV system and a VAV system is practical, the baseline system should be modeled as system 5 with the 0.4 cfm/ft2 requirement. It is important to note that the ASHRAE 90.1-2004 Standard Appendix G modeling protocol is not a compliance method but rather a method of comparing the proposed design\'s energy performance to a building that would have been typically built. Alternatively, the project may opt to model the VAV system as energy neutral and use the design minimum turn down ratios of the proposed design.