Sheffer has suggested the following document (see pp. 3.97-3.98):
http://www.pnnl.gov/main/publications/external/technical_reports/PNNL-25130.pdf
Consider this statement:
â??With parallel style fan-powered VAV boxes, the constant volume terminal unit fan is only on when the primary airflow is at design minimum and the zone temperature is less than 2°F above the heating setpoint schedule.â??
Therefore Iâ??m modeling in the following way.
The PIU boxes are always available. The â??Fan On Flow Fractionâ?? is equal to the â??Minimum Primary Air Flow Fractionâ?? (â??the constant volume terminal unit fan is only on when the primary airflow is at design minimumâ??â?¦).
AirTerminal:SingleDuct:ParallelPIU:Reheat,
FloorX09:OXOX1 Parallel PIU, !- Name
On, !- Availability Schedule Name
autosize, !- Maximum Primary Air Flow Rate {m3/s}
autosize, !- Maximum Secondary Air Flow Rate {m3/s}
0.3, !- Minimum Primary Air Flow Fraction
0.3, !- Fan On Flow Fraction
VAV09 Ramificazione di Zona Outlet Node 1, !- Supply Air Inlet Node Name
FloorX09:OXOX1 Parallel PIU PIU Secondary From Zone, !- Secondary Air Inlet Node Name
FloorX09:OXOX1 Parallel PIU Supply Inlet, !- Outlet Node Name
FloorX09:OXOX1 Parallel PIU PIU Mixer Outlet, !- Reheat Coil Air Inlet Node Name
FloorX09:OXOX1 Parallel PIU Zone Mixer, !- Zone Mixer Name
FloorX09:OXOX1 Parallel PIU Supply Fan, !- Fan Name
Coil:Heating:Electric, !- Reheat Coil Object Type
FloorX09:OXOX1 Parallel PIU Heating Coil, !- Reheat Coil Name
, !- Maximum Hot Water or Steam Flow Rate {m3/s}
, !- Minimum Hot Water or Steam Flow Rate {m3/s}
, !- Hot Water or Steam Inlet Node Name
0.0010; !- Convergence Tolerance
The PIU fans are never available, but they turn on if it is requested by the heating demand (nightcycle):
Fan:ConstantVolume,
FloorX09:OXOX1 Parallel PIU Supply Fan, !- Name
Off 24/7, !- Availability Schedule Name
0.825, !- Fan Total Efficiency
610, !- Pressure Rise {Pa}
autosize, !- Maximum Flow Rate {m3/s}
0.90, !- Motor Efficiency
1.00, !- Motor In Airstream Fraction
FloorX09:OXOX1 Parallel PIU PIU Secondary From Zone, !- Air Inlet Node Name
FloorX09:OXOX1 Parallel PIU Supply Fan Outlet, !- Air Outlet Node Name
PIU Supply Fan; !- End-Use Subcategory
As â??Thermostat Toleranceâ?? Iâ??m using a negative value because â??? and the zone temperature is less than 2°F above the heating setpoint scheduleâ?? (2°F is about 1°C). Actually, if I consider the results it seems that a negative value is considered like 0.
AvailabilityManager:NightCycle,
VAV09 UTA Ciclo Operativo Notturno 2, !- Name
On 24/7, !- Applicability Schedule Name
Off 24/7, !- Fan Schedule Name
CycleOnAnyZoneFansOnly, !- Control Type
-1, !- Thermostat Tolerance {deltaC}
3600; !- Cycling Run Time {s}
Moreover, Iâ??m using a AvailabilityManager:NightCycle object to control the cooling demand during unoccupied hours.