Background: Our project is a 51,250-sqft addition to an existing college building. The 90,100-sqft existing building houses offices, classrooms and laboratories in (3) 2-story wings built in 1966. The central plant for the existing building plus the addition consists of (2) equal size chillers and (3) equal size gas boilers on primary hot- and chilled-water loops. Each wing of the existing building is conditioned by its own air handler with hot- and chilled-water coils through three sets of secondary water loops. Usage history indicates that a maximum of one chiller and two boilers are operating at any given time. One chiller and one boiler are currently standbys and are large enough to handle the addition. The new addition will also house offices, classrooms and laboratories with the same operating schedules. It will be conditioned through (4) VAV units with hot- and chilled- water on a fourth set of secondary loops from the existing central plant. Questions: We would like to exclude the existing building from the energy model because of the time and expense to build and debug a model for this existing building that is nearly twice the size of the building addition being studied. ASHRAE 90.1-1999 Section 11.3.10.3 Alterations and Additions allows compliance using building models that exclude parts of the existing building provided (4) conditions are met. Our project meets conditions a, c, and d. Our question addresses condition b: "Excluded parts of the building are served by HVAC systems that are entirely separate from those serving parts of the building that are included in the building model". Since the addition is served by AHUs entirely separate from the existing building on its own secondary water loops, we propose two possible approaches wherein we model the addition only with a modified central plant. We would like a ruling on whether either of these approaches is acceptable: 1. Model the central plant with only the standby chiller and boiler on the primary loop with actual pump sizes decreased in proportion to the existing versus new square footage. The addition would then be modeled as normal using a set of secondary water loops with actual pump sizing. -or- 2. Model the existing central plant with both chillers, all three boilers, and actual pump sizes and add a scheduled process load on each primary loop based on the heat loss/gain calcs for the existing building to approximate the existing building loads on the equipment. The addition would then be modeled as normal using a set of secondary water loops with actual pump sizing.