The "Greening of Dana" project is a renovation of an historic building at the University of Michigan. The project retained some pre-existing mechanical systems and also installed new ones. This CIR seeks help applying the ASHRAE 90.1 rules for establishing the budget building systems in this context. The total Dana area is roughly 107,800 SF. Air handling unit 1 (AH-1) is a 100 % outdoor-air (OA) ventilation-only unit that was installed when 23,800 SF were added to the building two years before the current 84,000 SF project. AH-1 was designed to provide OA for both the addition and the rest of the building. The revised HVAC system for the Greening of Dana project uses pre-existing perimeter radiators for heating and a new radiant panel system for cooling. AH-1 was incorporated into the new system as it was originally intended, to provide OA throughout the building. Re-using AH-1 was both economically practical and logical for the design. The question that this context raises is, "What is the proper system configuration for the budget building energy model?" Specifically, "Should the budget building model include an economizer?" For existing buildings, ASHRAE 90.1 has conflicting requirements. In 11.1.3 Trade-Offs Limited to Building Permit it states that "[w]hen the building permit being sought applies to less than the whole building, only the calculation parameters related to the system to which the permit applies shall be allowed to vary. Parameters relating to unmodified existing conditions . shall be identical for both the energy cost budget and design energy cost calculations." By this logic, since AH-1 is existing, it makes sense that this system be modeled in both the budget and proposed simulations. However, in section 11.4.3 HVAC Systems, it states that the HVAC system type and design parameters for the budget building shall be determined by the system map and requires that budget systems shall have economizers in accordance with the requirements of section 6.3.1 Economizers. Section 6.3.1 states that for this climate (1% Twb of 73F and 640 hrs from 8am and 4pm where the Tdb is between 55F and 69F) economizers are required for all systems greater than 11 tons. (Note that the Twb and the Tdb hours in this climate barely require an economizer.) Due to this requirement, the budget building system should ostensibly be modeled with an economizer. However, due to the 11-ton limit, a fan-coil system serving the budget building with individual fan-coils of less than 11 tons each would be exempted from the economizer requirement. This is the situation we found at the start of the Dana project - multiple small fan-coils and no economizer. Further, if section 11.4.3 were to be followed, it is not clear how the requirements of section 11.1.3 would also be followed. The OA-only ventilation system was an existing to remain condition and we were directed by this section to model it the same in the budget and design case buildings. If we follow this rule, how could we then also simulate an economizer? The two systems are mutually exclusive. Finally, a number of factors preclude the use of a water-side economizer, including the historic nature of the building and campus location. To summarize, AH-1 was existing equipment before the Dana renovation project. Section 11.1.3 of ASHRAE 90.1 prescribes requirements to be followed in existing buildings. ASHRAE 90.1 includes many other requirements for all building types, most of which are new construction. The Dana team believes that section 11.1.3 requirements in ASHRAE 90.1, written specifically for existing buildings, take precedence over other more general conflicting requirements. We ask USGBC to rule that it is appropriate to model the budget systems without an economizer because this approach is a realistic and fair way to evaluate the energy performance of the project and because this approach is consistent with the intent of ASHRAE 90.1 when applied to existing buildings.