This group of building descriptors applies to proposed design systems that vary the volume of air at the zone level. The building descriptors are applicable for baseline building systems 5 through 8.
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Design Airflow |
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Applicability |
Systems that vary the volume of air at the zone level |
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Definition |
The air delivery rate at design conditions |
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Units |
cfm |
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Input Restrictions |
As designed. If the unmet load hours in the proposed design are greater than 300, the building descriptor may need to be modified to meet the criterion. The simulation for compliance purposes shall not proceed if the unmet load hours are greater than 300. |
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Baseline Rules |
For systems 5 through 8, the software shall automatically size the airflow to meet the baseline building loads based on the greater of the following approaches:
However, airflows for systems serving laboratory spaces shall be sized based on a suppluy air to room air temperature difference of 17 °F or the required ventilation air or makeup air, whichever is greater. |
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Terminal Minimum Stop |
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Applicability |
Systems that vary the volume of air at the zone level |
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Definition |
The minimum airflow that will be delivered by a terminal unit before reheating occurs |
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Units |
Unitless fraction airflow (cfm) or specific airflow (cfm/ft²) |
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Input Restrictions |
This input must be greater than or equal to the outside air ventilation rate. |
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Baseline Rules |
For systems 5 through 8, the minimum airflow for the VAV boxes should be 30% of zone the peak supply air volume; the outside air ventilation rate; or the air flow rate required to comply with applicable codes or accreditation standards, whichever is larger. Refer to section ‘Zone Exhaust’ for requirements related to systems serving laboratory spaces. |
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Terminal Heating Control Type |
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Applicability |
VAV boxes with reheat |
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Definition |
The control strategy for the heating mode. Single Maximum. In the single maximum control mode, the airflow is set to a minimum constant value in both the deadband and heating mode. This airflow can vary but is typically 30 to 50 percent of maximum. This control mode typically has a higher minimum airflow than the minimum used in the dual maximum below, resulting in more frequent reheat. Figure 3.6.5-1 Single Maximum Control SequenceCourtesy: Taylor Engineering Dual Maximum. This control sequence raises the SAT as the first stage of heating, and increases the airflow to the zone as the second stage of heating. 1. The first stage of heating consists of modulating the zone supply air temperature setpoint up to a maximum setpoint no larger than 95ºF while the airflow is maintained at the dead band flow rate. 2. The second stage of heating consists of modulating the airflow rate from the dead band flow rate up to the heating maximum flow rate (50% of design flow rate) while maintaining the maximum setpoint temperature. Figure 3.6.5-2: Dual Maximum Control SequenceCourtesy: Taylor Engineering |
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Units |
List either Single Maximum or Dual Maximum (see above) |
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Input Restrictions |
As designed |
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Baseline Rules |
Single Maximum at 30% of zone peak air flow or the outside air ventilation rate, or the air flow rate required to comply with applicable codes or accreditation standards, whichever is larger. Refer to the section ‘Zone Exhaust’ for requirements related to laboratory spaces. |
This group of building descriptors applies to proposed design systems that vary the volume of air at the zone level. The building descriptors are applicable for baseline building systems 5 through 8 and system 11.
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Design Airflow |
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|---|---|
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Applicability |
Systems 5 through 8 and 11 that vary the volume of air at the zone level |
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Definition |
The air delivery rate to each zone at design conditions |
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Units |
cfm |
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Input Restrictions |
As designed. If the UMLH in the proposed design are greater than 300, the simulation should not proceed and the user may have to modify the design airflow value. |
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Baseline Building |
For systems 5 through 8 and 11, the software shall automatically size the system airflow to meet the baseline building loads based on · a supply-air-to-room-air temperature difference of 17°F for systems serving laboratory spaces and 20°F for all other spaces · or the minimum outdoor airflow rate, · or the airflow rate required to comply with the applicable codes or accreditation standards, whichever is greater. The baseline system airflow is determined by the load to be met by the airflow and the 20°F (11°C) temperature difference. The loads to be used would be the design load as determined by the sizing runs specified in Section G3.1.2.2, not the cooling or heating capacity of the system as determined using the sizing factors, also specified in G3.1.2.2. Using the system cooling and heating capacity will result in oversized baseline system airflows and energy cost because of the oversizing factors used in G3.1.2.2. For zones served by baseline systems with multiple zone thermostat setpoints (systems 5 to 8), the design set points used should result in either the lowest supply air cooling setpoint or highest heating supply air heating setpoint. If the proposed design HVAC system airflow rate based on latent loads greater than the same based on sensible loads, then the same supply-air-to-room humidity ratio difference (gr/lb) used to calculate the proposed design airflow should be used to calculate the design airflow rates for the baseline building. For baseline systems 9 and 10, the design supply airflow rates shall be based on the temperature difference between a SAT setpoint of 105°F and the design space heating temperature setpoint, the minimum outdoor airflow rate or the airflow rate required to comply with applicable codes, whichever is greater. |
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Terminal Minimum Stop |
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Applicability |
Systems that vary the volume of air at the zone level |
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Definition |
The minimum airflow that will be delivered to a zone. For systems with reheat (system 5 to 8) the minimum flow before reheating occurs |
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Units |
Unitless fraction of airflow (cfm) or specific airflow (cfm/ft²) |
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Input Restrictions |
This input must be greater than or equal to the outside air ventilation rate |
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Baseline Building |
For systems 5 to 8, the minimum airflow for the VAV reheat boxes should be set to 30% of the zone peak supply air volume or the outside air ventilation rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger. Refer to the section Zone Exhaust for requirements related to systems serving laboratory spaces. For baseline system 11, minimum volume setpoint shall be 50% of the maximum design airflow rate, the minimum ventilation outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger. Fan volume shall be reset from 100% airflow at 100% cooling load to minimum airflow at 50% cooling load. In heating mode, supply air temperature shall be modulated to maintain space temperature and fan volume shall be fixed at the minimum airflow. Systems serving laboratory spaces shall reduce the exhaust and makeup air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards. |
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Terminal Heating Control Type |
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Applicability |
VAV boxes with reheat |
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Definition |
The control strategy for the heating mode. The nomenclature in simulation programs to simulate this may differ from that provided below. Single Maximum In the single maximum control mode, the airflow is set to a minimum constant value in both the deadband and heating mode. This airflow can vary but is typically 30% to 50% of maximum. This control mode typically has a higher minimum airflow than the minimum used in the dual maximum below, resulting in more frequent reheat.
Figure 6. Single Maximum VAV Box Control (Courtesy: Taylor Engineering)Dual Maximum: raises the SAT as the first stage of heating, and increases the airflow to the zone as the second stage of heating.
Figure 7. Dual Maximum Control Sequence |
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Units |
List: Single Maximum, Dual Maximum |
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Input Restrictions |
As designed |
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Baseline Building |
Single maximum at 30% of zone peak airflow or the outside air ventilation rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger. The ventilation outdoor air and exhaust/relief dampers should be shut-off during preoccupancy building warm-up, cool down and setback except for economizer operation. Refer to the section Zone Exhaust for requirements related to laboratory spaces. For baseline system 11, fan volume shall be reset from 100% airflow at 100% load to minimum airflow at 50% cooling load. The minimum airflow shall be 50% of the maximum design airflow rate, the minimum ventilation rate or the airflow required to comply with applicable codes or accreditation stations, whichever is larger. Systems serving laboratory spaces shall reduce the exhaust and makeup air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards. |
