The Natural Sciences Complex is a new three story 51,884 square foot, L-shaped building. The building includes college level science laboratories, laboratory support rooms, faculty offices, classrooms, and study areas. The HVAC system serving the laboratory wing of the building includes a single packaged VAV air handler with hot water reheat coils, a run-around heat recovery loop and a variable volume general exhaust system. In addition, laboratory spaces requiring fume hood exhaust are equipped with variable volume exhaust air devices mounted directly on top of fume hoods. The office wing is naturally ventilated and it is heated by a hydronic heating system. There are five classrooms that have mixed mode ventilation system and they are heated and cooled through the radiant slab. There are sixteen chemical fume hoods, and twelve chemical flow benches located in ten laboratories. Laboratory support rooms contain several storage cabinets that require continuous ventilation. Total fume hood exhaust rate is 11,600 cfm, and total exhaust rate of chemical flow benches is 2,100 cfm. Designed minimum ventilation rate for spaces containing chemicals is four ACH, as recommended by NFPA Standard 45. The same standard prohibits recirculation of chemicals originating from the laboratories. Since the 24,150 square feet laboratory wing contains only a few small spaces without chemicals (total floor area 450 square feet), the packaged air handler serving this area is 100% outside air system. This efficiently utilizes heat recovery system, reduces fan energy, and eliminates need for return air duct. The laboratory spaces are load driven. Total supply and exhaust flow rates for laboratory wing are 38,000 cfm and 33,000 cfm respectively. Following the requirements of the ASHRAE Standard 90.1-2004, Appendix G, Section G 3.1.1, based on usage, number of floors, conditioned floor area and heating source, our baseline model is a constant volume packaged single zone air conditioner with DX cooling coils and fossil fuel furnace. This type of system is assigned to each thermal zone. There is no thermal zone in the baseline model that has supply air capacity of 5,000 cfm or greater, and exhaust air energy recovery was not modeled in the baseline building according to Section G 3.1.2.10. This produced considerable energy savings due to the huge amount of outside air and the requirements for continuous ventilation of the laboratory spaces even during unoccupied hours. Since a large portion of our energy savings is the result of adding heat recovery to our system, we wanted to verify that we have correctly interpreted that the ASHRAE Standard does not require heat recovery in the baseline model for our building. Please advise if this modeling strategy is acceptable by USGBC.
The applicant requests confirmation that their modeling approach that excludes exhaust heat recovery from the baseline case due to baseline case system size is an acceptable modeling approach. This modeling approach is acceptable. The language of Section G3.1.2.10 with regards to system design airflow and outside air percentage refers to baseline case systems, not to proposed systems. As indicated above, the baseline case systems for the laboratory spaces all have air flow less than 5,000 cfm. Therefore, based on the description above, energy recovery would likely not be required in the baseline case systems serving the laboratory spaces. Please note that thermal blocks must have airflow less than 5,000 cfm for any thermal block where energy recovery is modeled for credit, and the size of the thermal block may not be manipulated to reflect a smaller thermal block than would typically be modeled just to avoid the prescriptive requirement for energy recovery.