2.6 Calculation Procedures

The general calculation procedure is illustrated in Figure 3 and explained in the steps below.

1. The process begins with a detailed description of the proposed design. Information is required to be provided in enough detail to enable an estimate of annual energy use for a typical weather year. This information includes the building envelope, the lighting systems, the HVAC systems, the water heating systems, and other important energy-using systems. This collection of information is referred to in this manual as building descriptors. Details on the building descriptors are provided in Chapter 2.
2. If the values of occupant density, equipment power density, ventilation rates, and water heating loads for the proposed building are not known, defaults based on the building type (as specified in Section 2.4.5.1) shall be used. Each building descriptor shall be either a user-defined input or the default value for that input where a default is available.
3. The next step is to simulate the proposed design to determine how well the heating and cooling loads are being satisfied. The indicator is the total number of UMLH. Test the number of UMLH and proceed only if the hours are less than or equal to 300 for the year of the proposed design simulation.
4. If the UMLH are greater than 300 for the year, then the user adjusts the proposed building simulation model to reduce the UMLH to less than or equal to 300. See Sections 2.5 and 2.7 for discussion on how UMLHs can be reduced.
5. If the UMLH are less than or equal to 300, then the final simulation is performed. If no changes are made in the model, the simulation from step 3 may be considered final. These calculations produce the results that are compared to the baseline building, which is calculated in steps 7 through 16.
6. The next steps relate to the creation of the baseline building model. The baseline building is created following the rules in this manual. It has the same floor area, number of floors, and spatial configuration as the proposed design; however, systems and components are modified to be in minimum compliance with Standard 90.1-2019 PRM. The HVAC systems for the baseline building are established according to rules in this manual and depend on the primary building activity (residential or non-residential), the floor area, and the number of stories. See Section 3.1.

Figure 3. Calculation Process for Standard 90.1-2019 Performance Cost Index Using Performance Rating Method

1. Sizing calculations are performed for the baseline building, and heating equipment is oversized by 25% and cooling equipment by 15%. A sizing run is done for each of the four orientations, and system properties (efficiency, controls, etc.) are specified based on equipment size determined through the corresponding sizing run. Equipment and plant capacities shall be based on coincident loads. This is further discussed in Section 2.7.2 of this manual.
2. The baseline building is simulated to determine the number of UMLH. This is the same as the process performed for the proposed design in step 3.
3. The number of UMLH for the baseline building is then tested to see if they are greater than 300. This is unlikely since the heating and cooling equipment is oversized by 25% for heating and 15% for cooling in step 7.
4. If the UMLH are greater than 300, then steps need to be taken to reduce the unmet hours to less than or equal to 300. See Sections 2.5 and 2.7 for discussion on how UMLHs can be reduced.
5. Once the tests on UMLH are satisfied, the energy consumption of the baseline building is calculated. If the tests on unmet hours are satisfied the first time through, this step is the same as step 9.
6. The baseline building is rotated 90 degrees and modeled again. This is repeated for four orientations. Each time the building is rotated the equipment is resized. This step may be omitted if the building orientation is dictated by the site or if fenestration area on each orientation varies by less than 5%.
7. The baseline energy use for the baseline building is calculated as the average of the energy use for the four orientations.
8. Finally, the performance cost index is calculated as the ratio of the proposed design energy cost and baseline building energy cost.

The next two steps are followed if the PRM is being used for determination of compliance with Standard 90.1.

1. The PCIt is calculated in accordance with Section 1.3.
2. The PCI is compared to the PCIt to determine if the proposed building design is in compliance with Standard 90.1.