2.4 Calculation Procedures

Not applicable for Design to Earn ENERGY STAR.

The process for tax deductions and green building ratings is illustrated in [bookref id="calculation-process-for-tax-deductions-and-green-building-ratings"]. For both of these purposes, the proposed design is compared to a baseline building and the percent savings are calculated for the proposed design relative to the baseline building.

[figure title="Calculation Process for Tax Deductions and Green Building Ratings" id="calculation-process-for-tax-deductions-and-green-building-ratings"]
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1. The process begins with a detailed description of the proposed design. Information is 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. A detailed presentation of the building descriptors are provided in Chapter 6.
2. Before the calculations are performed, some of the building descriptors are modified for the proposed design to incorporate prescribed (neutral independent) modeling assumptions. Prescribed modeling assumptions are different depending on purpose. For tax deductions, they include schedules of operation and plug loads. For green building ratings, there are few prescribed modeling assumptions.
3. The next step is to make a simulation of the proposed design to determine how well the heating and cooling loads are being satisfied. The indicator isunmet load hours, the number of hours during the year when the space temperature is below the heating set point temperature or greater than the cooling set point temperature. A large number of hours indicate that the equipment is undersized.
4. Test the number of unmet load hours and proceed only if the hours are less than 300 for the year of the simulation.
5. If the unmet load hours are greater than 300 for the year, then the baseline building simulation model is adjusted to reduce the unmet load hours to less than 300. If the problem is heating, then the size of the boiler or furnace may need to be increased. If the problem is cooling, then the size of the coils or chillers may need to be increased. These adjustments are not made automatically for the proposed buidling, but rather are specified by the modeler. See [bookref id="procedure-for-adjusting-equipment-hvac-sizes-in-the-baseline-building"].
6. If the unmet load hours are less than 300, then the final simulation is performed. If no changes are made in the model, this may be the same simulation in step 3. These calculations produce the results that are compared to the baseline building, which is calculated in steps 7 through 16.
7. Create the baseline building following the rules in this manual. The baseline building 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 the baseline standard. 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, the number of stories and the fuel used for heating. See [bookref id="hvac-mapping"].
8. Sizing calculations are performed for the baseline building and heating equipment is oversized by 25% and cooling equipment by 15%.
9. The next step is to make a simulation of the baseline building to determine how well the heating and cooling loads are being satisfied. This process is the same as performed for the proposed design in step 3.
10. The number of unmet load hours is then tested to see if they are greater than 300. This is not likely to occur since the heating and cooling equipment is oversized by 15% for cooling and 25% for heating in step 8.
11. If the unmet load hours are greater than 300, then equipment in the baseline building is increased so that the unmet hours are less than 300. See [bookref id="procedure-for-adjusting-equipment-hvac-sizes-in-the-baseline-building"].
12. Once both the baseline building and the proposed design have unmet load hours less than 300, they are compared to confirm that the unmet load hours for the proposed design are not greater than 50 more than the baseline building.
13. If the difference in unmet hours is greater than 50, then the equipment in the baseline building is educed in size so that the difference is less than or equal to 50. See [bookref id="procedure-for-adjusting-equipment-hvac-sizes-in-the-baseline-building"].
14. Once the tests on unmet load hours are satisfied, then the energy performance of the baseline building is calculated. If the tests of unmet hours are satisfied the first time through, this step is the same as step 9.
15. 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.
16. The baseline energy use for the baseline building is calculated as the average of the energy use for the four orientations.
17. The next step is to make a simulation of the proposed design to determine how well the heating and cooling loads are being satisfied. Finally, the percent savings are calculated. For tax deductions, only regulated energy is considered, but for green building ratings, total energy is considered.

The COMNET calculation process described above is consistent with the ASHRAE Standard 90.1 Performance Rating Method (PRM) as contained in Appendix G of the Standard.