2.5.2 Sizing Equipment in the Baseline Building

Equipment in the baseline building is automatically oversized by the program (25% for heating and 15% for cooling). However, in cases when unmet load hours in the proposed design are greater than 50 hours compared to the baseline building, equipment in the baseline building may have to be downsized. The criterion is that the unmet load hours in the proposed design may be no greater than 50 hours more than the corresponding thermal block in the baseline building. Figure 2.5.2-1 shows the recommended procedure for downsizing equipment in the baseline building so that the 50 hour delta requirement is satisfied. Note that this procedure may result in the baseline building equipment not meeting the 25% oversizing requirement for heating and the 15% oversizing requirement for cooling. It is also possible that the downsizing will result in a reduction in the 20 F delta-T specified in § G3.1.2.8 of the PRM.

Unmet load hours are evaluated at the building level by looking at the unmet load hours for each of the thermal blocks being modeled. One hour with unmet loads in one or more thermal block counts as a single unmet load hour for the building. Therefore, the unmet load hours for the building will never be less than the worst thermal block.

Figure 2.5.2-1 shows the process of adjusting equipment sizes in the baseline building in order to meet the 50 hour delta requirement. Equipment in the baseline building is already oversized, so the process is to incrementally make adjustments to the thermal block with the most unmet load hours until the un-met load hours for the baseline building are within 50 of the proposed design. The process is explained in greater detail in the paragraphs that follow Figure 2.5.2-1.

Figure 2.5.2-1: "Procedure for Adjusting Equipment HVAC Sizes in the Baseline Building"

1. The process begins with simulation results for both the proposed design and the baseline building.

2. Simulate the baseline building and calculate the un-met load hours for the building (see definition above).

3. Compare the unmet load hours between the proposed design and the baseline building. If the proposed design is no more than 50 hours greater than the baseline building, then move no adjustments are necessary and the process is complete (step 18), otherwise move to 4.

4. When the difference between the proposed design and the baseline building is greater than 50 unmet load hours, then determine the thermal block with the greatest total un-met load hours for heating and/or cooling. Adjustments will be made to this thermal block.

5. Test to see which is greater: the difference in heating unmet load hours or cooling unmet load hours.

6. If the difference for the thermal block is mostly cooling, then look at the system type serving the thermal block. If the system is type 1 through 4 (single zone systems) then go to 8, otherwise go to 9.

8. For system types 1 through 4, test to see if it is possible to reduce air flow to the thermal block and still maintain the minimum outside air ventilation level. If so, go to 10; otherwise, go to 11.

10. Reduce the cooling capacity of the packaged equipment by 2% and let the air flow to the zone scale in order to meet a 20°F delta-T difference between the setpoint temperature in the space and the supply air. Maintain the same ratio of sensible to total cooling capacity.

11. Raise the supply air temperature to the thermal block by 1°F. This reduces the 20°F delta-T, but is necessary to maintain air flow to the space at a volume adequate to meet the outside air ventilation requirement.

9. For system types 5 through 8, test to see if it is possible to reduce air flow to the thermal block and still maintain the minimum outside air ventilation level. If so, go to 12; otherwise, go to 13.

12. Reduce the design air flow rate to the thermal block by 2%. Allow the upstream coil and cooling equipment to be auto-sized so that their capacity is also reduced.

13. Raise the supply air temperature to the thermal block by 1°F. This reduces the 20°F delta-T, but is necessary to maintain air flow to the space at a volume adequate to meet the outside air ventilation requirement.

7. If the difference for the thermal block is mostly heating, then look at the system type serving the thermal block. If the system type is 1 or 3, go to 14; if the system type is 2 or 4, go to 15; if the system type is 5 or 6, go to 16; or if the system type is 6 or 8, go to 17.

14. For system types 1 or 3, reduce the heating coil capacity or the size of the furnace by 2%

15. For system types 2 or 4, reduce the size of the heat pump compressor and the supplemental heater by 2%. Note that for modeling purposes the size of the heat pump compressor is changed without changing the size of the cooling capacity.

16. For system types 5 or 6, reduce the size of the reheat coil by 2%. Auto size the boiler accordingly.

17. For system types 6 or 8 reduce the size of the electric reheat coil by 2%.

3. Move back to step 2.

20. Finish.