While many engineers and architects throughout the United States have used ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, as the basis of their building design, there remains some confusion about the proper application and requirements of the standard, especially surrounding the Energy Cost Budget Method (ECBM). As ASHRAE prepares to release the 2004 edition of 90.1, let's take a quick review of the standard.

HISTORY OF STANDARD 90.1

Standard 90.1 originated in 1975 in response to the energy crisis of the early 1970s. It was revised in 1980 (taking longer than the usual ASHRAE standards cycle of the time), but became more prominent in building designs with the co-sponsorship of the Illuminating Engineering Society of North America (IESNA) in the 1989 edition and as a result of its adoption within the building codes of many regions of the country.

In 1999, ASHRAE placed the standard on continuous maintenance in conjunction with the American National Standards Institute (ANSI). The U.S. Department of Energy (DOE) also reviewed the 1999 edition and determined that it was in the country's best interests to require all states to implement a state energy code requirement that met or exceeded Standard 90.1-1999 by July 2004 (see sidebar).

With this increased interest in Standard 90.1-1999, the U.S. Green Buildings Council, through its Leadership in Energy and Environmental Design (LEED) rating system, created the Green Building Design Program, which provides up to 10 points (out of the 26 needed for basic LEED certification) in the LEED-NC credit for optimizing energy performance (Credit EA-1).

Chapter 11 of Standard 90.1 deals with the ECBM. This may be the least understood portion of the standard. ECBM is a critical requirement of LEED-NC Credit EA-1. Standard 90.1 now requires (per addendum P) that the energy simulation program utilized be tested in accordance with ANSI/ASHRAE Standard 140-2001, Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs. Therefore, any energy simulation program utilized should have this test documentation.

As with most things in life, the best way to learn is to do. The purpose of this article is to provide a place to start and a “big” picture to aid engineers in utilizing ECBM in design practice. A review of the standard's user's manual, which is available from ASHRAE, is recommended to clarify the procedures to properly implement the ECBM calculation.

ENERGY COST BUDGET METHOD

The ECBM is a comparative modeling system in which two models of the same building are created and compared.

• The first model is the “Proposed Design Model” (PDM) (also referred to as the “Design Energy Cost”). This is the building as designed. It is exactly what the construction documents (or contract drawings and specifications) show. The building occupancy, envelope, areas (floor, windows, walls, roof, and skylights), and MEP systems (HVAC, lighting, power and, plumbing systems) are included in this model.

• The second model is the “Budget Building Design” (BBD) (also referred to as the “Energy Cost Budget”). Just like the PDM, this is the building the engineer would like to construct; however, this model uses 90.1's prescriptive (minimum) requirements for component performances. This model has exactly the same floor and envelope dimensions as the construction documents show. The envelope components, while dimensionally the same, have the code minimum performance values (i.e., insulation of walls, roof, and glazing). The building occupancy and MEP systems are also included in this model. However, the MEP systems can be modified to reflect the code minimum component efficiencies (with the same components). Controls for the lighting and HVAC systems should be the minimum required. The standard does acknowledge that some components, systems, or operational modes may not be properly simulated by an otherwise approved program. Therefore, 90.1 allows:

“Where no simulation program is available that adequately models a design, material, or device, the authority having jurisdiction may approve an exceptional calculation method to be used to demonstrate compliance with Section 11.”

If the design calls for a demand ventilation control strategy, one would have to get the authority having jurisdiction's (AHJ) approval for an exceptional calculation method to show the energy savings. It would then also be up to the LEED submittal review process to also approve for any additional Credit EA-1 points. Securing AHJ approval is a recommended step in the LEED review process.

It is important to understand that Standard 90.1 has specific requirements for the comparative models in terms of items that must be identical, such as the occupancy schedule, conditioned areas, temperature setpoints, building orientation, minimum outdoor air ventilation rates, etc. Likewise, LEED Credit EA-1 has Energy Modeling Protocol (EMP) requirements that can differ from those in the standard. It is very helpful to refer to the LEED credit interpretations for Credit EA-1, which are available to all registered project teams, to better understand EMP. Both LEED Credit EA-1 and 90.1 require the same simulation program, weather data, and energy rates be used for both models.

In addition, both the standard and LEED are most interested in the regulated (vs. non-regulated) energy, although only the LEED credit uses these terms directly. Non-regulated loads are “…energy used primarily to provide industrial, manufacturing, or commercial processes,” according to LEED-NC.

Note that the applicable non-regulated loads are included in the models and then subtracted from the overall building energy usage. This is done to properly model the MEP systems' actual performance (i.e., part loading) if, for example, there is a non-regulated load that impacts an air-handling system or a chiller plant.

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