2.5 HVAC Capacity Requirements and Sizing

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 baseline building. The subsections below describe the procedures that shall be followed to ensure that both the baseline building and the rated building are simulated with adequate space-conditioning capacities.

2.5.1 Specifying HVAC Capacities

As shown in Figure 2.4-1, the proposed design shall have no more than 300 unmet load hours. If this requirement is violated, the user shall make changes to the proposed design building description to bring the unmet load hours below 300. This process is not automated by the software. There are two tests that must be met:

  • Space loads must be satisfied: Space temperatures must be maintained within one half of the throttling range (e.g., 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 300 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 300 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 and appropriate changes shall be made to the construction documents 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 added by the user. The type of equipment added will depend on the type of HVAC system in the proposed design and the judgment of the energy analyst.

Equipment sizes for the proposed design shall be entered into the model by the energy modeler 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 block(s), the user shall be prompted to make changes to equipment sizes and to make corresponding changes to the construction documents. This occurs when the unmet load hours exceed 300 for the year.

Equipment in the baseline building is oversized (25% for heating and 15% for cooling).