The Performance Rating Method and this manual use the term “unmet load hours” as a criterion for sizing equipment, for qualifying natural ventilation systems, and for other purposes. The concept of unmet load hours applies to individual thermal blocks but is summed for the building as a whole. For a thermal block, it represents the number of hours during a year when the HVAC system serving the thermal block is unable to maintain the set point temperatures for heating and/or cooling. During periods of unmet loads, the space temperature drifts above the cooling setpoint or below the heating setpoint. An unmet load hour occurs only during periods when the HVAC system is scheduled to operate. One hour with un-met loads in one or more thermal block counts as a single un-met load hour for the building. If unmet load hours for more than one thermal block coincide (occur at the same hour), they count as only one unmet load hour for the building. Un-met load hours include periods when the space is either under cooled or under heated.
Unmet load hours can occur because fans, air flows, coils, furnaces, air conditioners or other equipment is undersized. Unmet load hours can also occur due to user errors including mismatches between the thermostat setpoint schedules and HVAC operating schedules or from other input errors, for instance, high internal gains or occupant loads. The term, as used in this manual, only addresses equipment undersizing. It is the responsibility of the user to address other causes of unmet load hours in the proposed design. There can be many reasons, but the following checklist is offered as a starting point:
- Make sure that thermostat schedules agree with schedules of HVAC system operation; occupant schedules; miscellaneous equipment schedules; outside air ventilation schedules and other schedules of operation that could affect the ability of the HVAC system to meet loads in the thermal block.
- Check to make sure that inputs for internal gains, occupants, outside air ventilation are reasonable and are consistent with the intended operation of the building.
- Examine the simulated operation of controls to determine if primary or secondary heating or cooling equipment (pumps, coils, boilers, etc.) is activated. Verify that the controls are not resetting in a way that reduces modeled capacity.
2.3.1 Definitions
A space is a subcomponent of an HVAC zone that has values identified for lighting, outdoor air ventilation, occupancy, receptacle loads, and hot water consumption requirements. A space could be conditioned, semi-heated, or unconditioned. An HVAC zone may contain more than one space type.
A Heating Ventilation and Air Conditioning (HVAC) zone is a space or collection of spaces within a building having space conditioning requirements that are similar enough to be maintained with a single thermal controlling device. An HVAC zone is a thermal and not a geometric concept: spaces need not be contiguous to be combined within a single HVAC zone. However, daylighting requirements may prevent combining non-contiguous spaces into a single HVAC zone. If individual spaces are not modeled but combined into a zone, the space type breakdown (floor area of each space) should be provided.
A thermal block is a collection of one or more HVAC zones grouped together for simulation purposes. Spaces need not be contiguous to be combined within a single thermal block.
Where HVAC Zones have been designed:
Similar HVAC zones can be combined into a single thermal block provided all of the following conditions are met:
- Are served by the same HVAC system or the same type of HVAC system
- The space use classification is the same throughout the thermal block or all of the zones have peak coincident internal loads that differ by less than 10 Btu/h·ft2 from the average.
- Have the same occupancy, equipment, lighting, and thermostat schedules and setpoints
- All HVAC zones in the thermal block that are adjacent to glazed exterior walls and glazed semiexterior walls face the same orientation or their orientations vary by less than 45 degrees.
- All of the zones have schedules that differ by 40 or less equivalent full-load hours per week
Residential spaces shall be modeled using at least one thermal block per dwelling unit, except that those units facing the same orientations may be combined into one thermal block. Corner units and units with roof or floor loads shall only be combined with units sharing these features.
Where the HVAC zones and systems have not yet been designed:
Thermal blocks shall be defined based on similar internal load densities, occupancy, lighting, thermal and space temperature schedules, and in combination with the following guidelines:
- Separate thermal blocks shall be assumed for interior and perimeter spaces. Interior spaces shall be those located greater than 15 ft from an exterior wall. Perimeter spaces shall be those located within 15 ft of an exterior wall.
- Separate thermal blocks shall be assumed for spaces adjacent to glazed exterior walls; a separate zone shall be provided for each orientation, except that orientations that differ by less than 45° may be considered the same orientation. Each zone shall include all floor area that is 15 ft or less from a glazed perimeter wall, except that floor area within 15 ft of glazed perimeter walls having more than one orientation shall be divided proportionately between zones.
- Spaces having floors that are in contact with the ground or exposed to ambient conditions shall have separate thermal blocks from zones that do not share these features.
- Floors with identical thermal blocks can be grouped for modeling purposes.
- Spaces having exterior ceiling or roof assemblies shall have separate thermal blocks from zones that do not share these features.
- Plenums are spaces typically located above the ceiling and/or below the floor above used to transfer air, where lighting fixtures, pipes, ducts and other building services are often located. Plenums are often used as part of a building’s return air pathway. Because of the leakage through the ceiling (typically suspended with lay-in panels), the temperature of the plenum tracks the temperature of the space, except that it is generally warmer because of heat stratification and heat produced by lighting fixtures located at the ceiling or in the plenum.
It is generally recommended that plenums be modeled as separate thermal blocks, but at the modeler’s discretion, they may be combined with conditioned space below for modeling simplicity. Figure 2 shows the hierarchy of spaces and HVAC zones.
|
|
Figure 2. Hierarchy of Space, HVAC Zones, and Thermal Block