Background: Our project is a 3 story, 16,500-sq.ft. addition to an existing 3 story, 84,000 sq.ft. building. The existing building is predominately laboratory space with some office space. The addition will be of similar use. The heating and cooling of the existing building is served by a central utility plant which provides chilled water and hot water via a steam boiler and heat exchanger. It is proposed that the addition also be served by the central plant. The central plant serves several other buildings on the site as well. In order to make a decision on whether we would like to obtain LEED registration on just the new addition or on the entire building with the new addition, a preliminary building simulation is being modeled. For the ASHRAE baseline, the system is modeled as a "System 3 - PSZ-AC" (packaged rooftop, constant volume, direct expansion, and fossil fuel furnace) per table G3.1.1A of ASHRAE 90.1-2004. Though the combined building size would categorize building with addition as a "System 5 - Packaged VAV w/ Reheat.", section G3.1.1(c) mandates conforming to the requirements of System 3 as an exception due to the special pressurization relationship/ cross-contamination requirement of the laboratory. Interpretation Request: Little is stated in ASHRAE 90.1 2004 on the most appropriate way to model a system that has chilled water and hot water heat supplied from a central plant. However, there are a few CIRs concerning similar circumstance that allude to it such as the 1/27/2004-2/24/2004 EA1.1 CIR. In it, it is stated that "While the situation described is not using purchased chilled water or steam, this HVAC description for the budget building is the closest to the proposed design and should be used for the energy modeling purposes." This approach for the budget building model is quite workable since the building owner has costs available for both chilled water and heating hot water. However, the baseline is modeled as a DX cooling and gas fired furnace. Is it appropriate to model the budget building with chilled water and heating hot water, when the baseline model is using neither of these? If not, how should the baseline and budget building be modeled?