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; for example, 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.
The UMLH requirement may be updated to prevent HVAC systems that show as undersized according to compliance software modeling constraints from receiving a compliance credit. This does not mandate that a specific cooling or heating capacity be specified; that is the role of the engineer of record for the building. With this change, if the proposed design appears as undersized, the user will be prompted to adjust plant, system, and/or zonal capacities as needed to meet the UMLH criteria.
The special case of a building designed with no cooling system (typically, in a temperate coastal climate) is accommodated by the compliance software automatically adding a minimally compliant packaged constant volume single zone system.
As shown in Figure 2, the proposed design shall have no more than 150 unmet load hours for any thermal zone. If this limit is exceeded, the software allows 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 to avoid excess unmet load hours:
•Space loads must be satisfied: Space temperatures in all thermal 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 percent for heating and 15 percent for cooling). If the automatic oversizing percentage is not sufficient to meet demands, then UMLH are evaluated at the building level by looking at the UMLH for each of the thermal zones being modeled. The zone with the greatest number of UMLH shall not exceed 150.
If the total number of UMLH 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 UMLH are the bigger problem. If heating UMLH 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 UMLH (greater than 150 for a zone) by 10 percent, then the equipment capacity for the system(s) serving the affected zones is increased to handle the increased zone loads. For a central plant, the chiller(s) and towers are resized proportionally to handle the increased system loads.
If heating is undersized, the same procedure is followed with zone terminal units resized first, heating equipment second, and finally boilers as necessary.
The capacity of the boiler or furnace shall be increased in 5 percent increments and the simulation rerun 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.
This section is under review, to ensure that designs for buildings with no cooling system specified in temperate climates can be accommodated in the performance compliance approach.
If a compliance model does not contain an HVAC system and 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 to identify that the building has no cooling system, this information is reported on the compliance reports.