VAV Air Flow
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.
Design Airflow
|
| Applicability |
Systems that vary the volume of air at the zone level |
| Definition |
The air delivery rate at design conditions |
| Units |
cfm |
| 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. |
| Baseline Rules |
For systems 5 through 8, the software shall automatically size the airflow to meet the baseline building loads based on a supply-air-to-room-air temperature difference of 20°F or the required ventilation air or makeup air, whichever is greater
|
Terminal Minimum Stop
|
| Applicability |
Systems that vary the volume of air at the zone level |
| Definition |
The minimum airflow that will be delivered by a terminal unit before reheating occurs |
| Units |
Unitless fraction airflow (cfm) or specific airflow (cfm/ft²) |
| Input Restrictions |
This input must be greater than or equal to the outside air ventilation rate. |
| Baseline Rules |
For systems 5 through 8, set the minimum airflow to be the greater of 0.4 cfm/ft² of conditioned floor area or the outside air ventilation rate. |
Fan Powered Boxes
Fan Powered Box Type
|
| Applicability |
HVAC zones that have fan powered boxes |
| Definition |
Defines the type of fan-powered induction box. This is either:
|
| Units |
List (see above) |
| Input Restrictions |
As designed |
| Baseline Rules |
Applicable for baseline building systems 6 and 8 and the fan powered box type is parallel.
|
Fan Power
|
| Applicability |
HVAC zones that have fan powered boxes |
| Definition |
The rated power input of the fan in a fan-powered box. |
| Units |
W or W/cfm |
| Input Restrictions |
As designed |
| Baseline Rules |
For baseline building systems 6 and 8, power is prescribed at 0.35 W/cfm. |
Fan Power Box Induced Air Zone
|
| Applicability |
HVAC zones that have fan powered boxes |
| Definition |
Zone from which a series or parallel fan-powered box draws its air |
| Units |
List (of zones) |
| Input Restrictions |
As designed |
| Baseline Rules |
If the proposed design has a plenum, then induced air will be drawn from that plenum. If the proposed design does not have a plenum, then induced air will be drawn from the space.
|
Parallel PIU Induction Ratio
|
| Applicability |
HVAC zones that have fan powered boxes |
| Definition |
The ratio of induction-side airflow of a fan-powered box at design heating conditions to the primary airflow |
| Units |
Ratio |
| Input Restrictions |
As designed |
| Baseline Rules |
50% |
Parallel Fan Box Thermostat Setpoint
|
| Applicability |
HVAC zones that have parallel fan powered boxes |
| Definition |
The temperature difference above the heating setpoint at which the parallel fan is turned on |
| Units |
Degrees Fahrenheit (°F) |
| Input Restrictions |
2°F above the heating setpoint schedule |
| Baseline Rules |
2°F above the heating setpoint schedule
|
Zone Exhaust
This group of building descriptors describes the rate of exhaust and the schedule or control for this exhaust. An exhaust system can serve one thermal block or multiple thermal blocks. Energy is summed for the exhaust system level, not the thermal block level.
Exhaust Fan Name
|
| Applicability |
All HVAC zones |
| Definition |
A reference to an exhaust fan system that serves the thermal block |
| Units |
Text or other unique reference to an exhaust fan system defined in the secondary systems section. |
| Input Restrictions |
As designed |
| Baseline Rules |
Same as the proposed design |
Exhaust Air Flow Rate
|
| Applicability |
All HVAC zones |
| Definition |
Rate of exhaust from a thermal block |
| Units |
cfm |
| Input Restrictions |
As designed |
| Baseline Rules |
Same as the proposed design |
Exhaust Fan Schedule
|
| Applicability |
All HVAC zones |
| Definition |
Schedule indicating the pattern of use for exhaust air from the thermal block. This input should consider the position of fume hood sash opening. For toilets and other exhaust applications, the schedule may coincide with the operation of the exhaust fan system. |
| Units |
Data structure: schedule, fraction |
| Input Restrictions |
As designed |
| Baseline Rules |
Same as the proposed design |
Outdoor Air Ventilation
Ventilation Source
|
| Applicability |
All HVAC zones |
| Definition |
The source of ventilation for an HVAC system. The choices are:
- Natural (by operable openings)
- Mechanical (by fan)
|
| Units |
List: natural or mechanical
|
| Input Restrictions |
For residential units and hotel/motel guest rooms, as designed. For all other occupancies, set to mechanical.
|
| Baseline Rules |
For residential units, set to natural, for all other occupancies, set equal to the value for the proposed design (mechanical). |
Design Ventilation Rate
|
| Applicability |
All HVAC zones |
| Definition |
The quantity of ventilation air that is provided to the space for the specified thermal block at maximum occupancy |
| Units |
cfm or cfm/occupant |
| Input Restrictions |
For the purpose of tax deduction calculations, the California ACM values from Table 4 of Appendix B shall be used. For green building ratings and Design to Earn ENERGY STAR, the COMNET values from Table 4 of Appendix B shall be used as a default. Other values may be used with appropriate documentation. |
| Baseline Rules |
Same as the proposed design |
Minimum Ventilation Rate
|
| Applicability |
All HVAC zones |
| Definition |
The minimum quantity of ventilation air that must provided to the space when the HVAC system is operating
|
| Units |
cfm or cfm/ft² |
| Input Restrictions |
As designed |
| Baseline Rules |
Same as the proposed design |
Ventilation Control Method
|
| Applicability |
All HVAC zones |
| Definition |
The method used to determine outside air ventilation needed for each hour in the simulation. This information is reported to the system serving the zone. The method of controlling outside air at the system level in response to this information is discussed under secondary systems. Options at the zone level are:
- Occupant sensors: When the space is occupied, the outside air requirement is equal to the design ventilation rate, otherwise, the outside air requirement is the minimum ventilation rate.
- CO2 sensors in the space: The outside air is determined to maintain a maximum CO2 concentration in the space. This may be approximated by multiplying the ventilation rate per occupant times the number of occupants for that hour.
- Turnstiles to determine the number of occupants in the space (appropriate for theatres, etc.): The outside air requirement is modulated based on the number of occupants in the space, based on the turnstile counts.
- Fixed ventilation rate. Outside air is delivered to the zone at a constant rate and is equal to the design ventilation rate (see above).
|
| Units |
List (see above) |
| Input Restrictions |
As designed |
| Baseline Rules |
For the federal tax credit, if the design occupancy is greater than 100 persons per 1,000 ft² and the system design outside air flow rate is greater than 3,000 cfm, set control method to CO2 sensors in the space, otherwise set to fixed ventilation rate.
For green building incentive programs, set to CO2 sensors in the space if the design occupancy is greater than 40 people per 1,000 ft² and the system design outdoor air flow rate is greater than 1,200 cfm, otherwise set to fixed ventilation rate.
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