Fan-free UnitarySystem on branch shuts down flow if No Load Supply Air Flow Rate is zero

[mjwitte]

Issue overview

Airloop with the following branch configuration never runs (fan power always zero):

Branch,
  Perim_1_WSHP Main Branch,               !- Name
  ,                                       !- Pressure Drop Curve Name
  AirLoopHVAC:OutdoorAirSystem,           !- Component Object Type 1
  Perim_1_WSHP OACtrl OA System,          !- Component Name 1
  Perim_1_WSHP Supply Side (Return Air) Inlet Node, !- Component Inlet Node Name 1
  Perim_1_WSHP_CoilClg Unitary Inlet Node, !- Component Outlet Node Name 1
  AirLoopHVAC:UnitarySystem,              !- Component Object Type 2
  Perim_1_WSHP_CoilClg Unitary,           !- Component Name 2
  Perim_1_WSHP_CoilClg Unitary Inlet Node, !- Component Inlet Node Name 2
  Perim_1_WSHP_CoilClg Unitary Outlet Node, !- Component Outlet Node Name 2
  AirLoopHVAC:UnitarySystem,              !- Component Object Type 3
  Perim_1_WSHP_CoilHtg Unitary,           !- Component Name 3
  Perim_1_WSHP_CoilClg Unitary Outlet Node, !- Component Inlet Node Name 3
  Perim_1_WSHP_CoilHtg Unitary Outlet Node, !- Component Outlet Node Name 3
  Fan:VariableVolume,                     !- Component Object Type 4
  Perim_1_WSHP_Fan,                       !- Component Name 4
  Perim_1_WSHP_CoilHtg Unitary Outlet Node, !- Component Inlet Node Name 4
  Perim_1_WSHP_Fan Outlet Node;           !- Component Outlet Node Name 4

One small change in both UnitarySystems makes it run.

in v9.4, the system ran fine with either set of inputs.

Details

Some additional details for this issue (if relevant):

• Version of EnergyPlus 25.1.0-9c7c2a6d79
• Helpdesk ticket number 16739

Checklist

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

• [x] Defect file added EnergyPlusDevSupport\DefectFiles
• [ ] 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)