To ensure that the simulated space-conditioning loads are adequately met, adequate capacity must be available in each of the components of the HVAC system; e.g., supply-air flow rates, cooling coils,
chillers, and cooling towers. If any component of the system is incapable of adequate performance, the simulation may understate the required energy inputs for space conditioning and report unmet load hours. Adequate capacities are required in the simulations of both the proposed design and standard design. The subsections below describe the procedures that shall be followed to ensure that both versions of the design are simulated with adequate space-conditioning capacities.
As shown in Figure 2, the proposed design shall have no more than 150 unmet load hours. If this requirement is violated, the software shall require the user to make changes to the proposed design building description to bring the unmet load hours equal to or below 150. This process is not automated by the software. There are two tests that must be met:
•Space loads must be satisfied: Space temperatures in all zones must be maintained within one half of the throttling range (1°F with a 2°F throttling range) of the scheduled heating or cooling thermostat setpoints. This criterion may be exceeded for no more than 150 hours for a typical year.
•System loads must be satisfied: Plant equipment must have adequate capacity to satisfy the HVAC system loads. This criterion may be exceeded for no more than 150 hours for a typical year.
If either the space or system loads do not meet the above criteria, the equipment in the proposed design shall be resized by the user such that the criteria are met. If the space conditioning criteria are not met because the HVAC equipment in the proposed design lacks the capability to provide either heating or cooling, equipment capable of providing the needed space conditioning must be specified by the user.
Equipment sizes for the proposed design shall be entered into the model by the energy analyst and shall agree with the equipment sizes specified in the construction documents. When the simulations of these actual systems indicate that specified space conditions are not being adequately maintained in one or more thermal zone(s), the user shall be prompted to make changes to equipment sizes or zones as necessary. This occurs when the unmet load hours exceed 150 for the year. The use of equipment sizes that do not match the actual equipment sizes as indicated on construction documents triggers an Exceptional Condition that is noted on the compliance forms.
For sizing heating and cooling equipment capacities, the compliance software shall use design day schedules as specified in Section 5.3. For cooling capacity sizing, compliance software shall use the OnDay schedule from Appendix 5.4B for occupant, lighting and equipment schedules, respectively. For heating capacity sizing, compliance software shall use the OffDay schedule from Appendix 5.4B for occupant, lighting and equipment schedules, respectively.
Equipment in the standard design is automatically oversized by the program (25% for heating and 15% for cooling). If the automatic oversizing percentage is not sufficient to meet demands, then Unmet load hours are evaluated at the building level by looking at the unmet load hours for each of the thermal zones being modeled. The zone with the greatest number of unmet load hours shall not exceed 150.
If the number of total unmet load hours for cooling and/or heating exceeds 150, then equipment capacities of cooling and/or heating equipment must be increased by the software incrementally.
1. The first step is to determine whether heating or cooling unmet load hours are the bigger problem. If heating unmet load hours are the bigger problem, upsize the heating equipment capacity. If cooling UMLH is the problem, upsize the cooling equipment capacity.
2. If the cooling is undersized, the equipment is resized by first increasing the design airflow of all zones with significant unmet load hours (greater than 150 for an individual zone) by 10%, i. Then the equipment capacity for the system(s) serving the affected zones is increased to handle the increased zone loads. For central plant the chiller(s) are towers are resized proportionally to handle the increased system loads.
If heating is undersized, the same procedure is followed, with zones resized first, then heating secondary equipment and then boilers as necessary.
The capacity of the boiler or furnace shall be increased in 5% increments and the simulation re-run until the loads are met. For heat pumps the capacity of the coil is increased so that the additional load is not met by auxiliary heat.
If a compliance model does not contain an HVAC system, if the number of unmet load hours exceeds 150 hours for any zone, then the compliance software shall prompt the user to enter a cooling capacity for an HVAC cooling system. The system type and efficiency characteristics shall match that of the standard design system. The compliance software shall make an appropriate note on compliance documentation indicating that the modeled HVAC system does not match design requirements. If the compliance software provides a means for the user that the building has no cooling system, then this information should be reported on the compliance reports or forms.