AirTerminal:SingleDuct:VAV:Reheat using ReverseWithLimits Damper Heating Action shows Maximum Flow Fraction During Reheat < 1

[rraustad]

Issue overview

Testing of VAV box using Damper Heating Action = ReverseWithLimits shows that a zone with heating dominated air flow rate has a Maximum Flow Fraction During Reheat < 1. This seems incorrect given that the VAV box should open to the full heating air flow rate when there is a heating load.

Defect file VAV terminal unit design:

AirTerminal:SingleDuct:VAV:Reheat,
  SPACE1-1 VAV Reheat,     !- Name
  ReheatCoilAvailSched,    !- Availability Schedule Name
  SPACE1-1 Zone Coil Air In Node,  !- Damper Air Outlet Node Name
  SPACE1-1 ATU In Node,    !- Air Inlet Node Name
  autosize,                !- Maximum Air Flow Rate {m3/s}
  Constant,                !- Zone Minimum Air Flow Input Method
  0.3,                     !- Constant Minimum Air Flow Fraction
  ,                        !- Fixed Minimum Air Flow Rate {m3/s}
  ,                        !- Minimum Air Flow Fraction Schedule Name
  Coil:Heating:Water,      !- Reheat Coil Object Type
  SPACE1-1 Zone Coil,      !- Reheat Coil Name
  autosize,                !- Maximum Hot Water or Steam Flow Rate {m3/s}
  0.0,                     !- Minimum Hot Water or Steam Flow Rate {m3/s}
  SPACE1-1 In Node,        !- Air Outlet Node Name
  0.001,                   !- Convergence Tolerance
  ReverseWithLimits,                  !- Damper Heating Action
  AUTOCALCULATE,           !- Maximum Flow per Zone Floor Area During Reheat {m3/s-m2}
  AUTOCALCULATE;           !- Maximum Flow Fraction During Reheat

The test file shows these results for zone design loads and air flow rates:

Zone Sizing Information, SPACE1-1, Cooling, 1991.75827, 1991.75827, 0.16753, 0.16753
Zone Sizing Information, SPACE1-1, Heating, 3970.36898, 3970.36898, 0.26058, 0.26058 <- heating dominated
Zone Sizing Information, SPACE2-1, Cooling, 2110.05356, 2110.05356, 0.17747, 0.17747
Zone Sizing Information, SPACE2-1, Heating, 1635.59910, 1635.59910, 0.10735, 0.10735
Zone Sizing Information, SPACE3-1, Cooling, 1822.43896, 1822.43896, 0.15328, 0.15328
Zone Sizing Information, SPACE3-1, Heating, 3862.85865, 3862.85865, 0.25352, 0.25352 <- heating dominated
Zone Sizing Information, SPACE4-1, Cooling, 2411.91565, 2411.91565, 0.20286, 0.20286
Zone Sizing Information, SPACE4-1, Heating, 1630.01391, 1630.01391, 0.10698, 0.10698
Zone Sizing Information, SPACE5-1, Cooling, 1144.55309, 1144.55309, 9.62850E-002, 0.13906
Zone Sizing Information, SPACE5-1, Heating, 3259.03012, 3259.03012, 0.21389, 0.21389 <- heating dominated

It would seem that for SPACE1, 3 and 5 that the Maximum Flow Fraction During Reheat would be 1 yet the results show otherwise:

Component Sizing Information, AirTerminal:SingleDuct:VAV:Reheat, SPACE1-1 VAV REHEAT, Design Size Maximum Flow Fraction during Reheat [], 0.77325
Component Sizing Information, AirTerminal:SingleDuct:VAV:Reheat, SPACE2-1 VAV REHEAT, Design Size Maximum Flow Fraction during Reheat [], 0.79773
Component Sizing Information, AirTerminal:SingleDuct:VAV:Reheat, SPACE3-1 VAV REHEAT, Design Size Maximum Flow Fraction during Reheat [], 0.77329
Component Sizing Information, AirTerminal:SingleDuct:VAV:Reheat, SPACE4-1 VAV REHEAT, Design Size Maximum Flow Fraction during Reheat [], 0.69789
Component Sizing Information, AirTerminal:SingleDuct:VAV:Reheat, SPACE5-1 VAV REHEAT, Design Size Maximum Flow Fraction during Reheat [], 1.00000

This is likely due to the terminal unit considering:

1. that the maximum air flow rate is the larger of heating and cooling, and more importantly,
2. that the heating air flow rate is limited by other information provided in the simulation (i.e., the "limited" heating air flow rate is less than the zone design heating air flow rate).

Issue provided by Trane North America.

Details

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• Platform (Operating system, version)
• Version of EnergyPlus (if using an intermediate build, include SHA)
• Unmethours link or helpdesk ticket number

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• Defect file added (5ZoneAirCooled-ReverseWithLimits.txt)
• Ticket added to EnergyPlus Defect Complexity (Github Project)
• Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)