How to add a preheat coil before heat exchanger?

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You can add coils before OAM (OutdoorAir:Mixer). You can refer to the ExampleFile: TermRhGenericOAHeatRecPreheat.idf in your PC.

The order of componets in the ExampleFile is

(Outdoor air) -> Preheat coil -> HEX -> OAM -> Fan -> Cooling coil

The preheat coil and the HEX are listed in AirLoopHVAC:OutdoorAirSystem:EquipmentList.

  AirLoopHVAC:OutdoorAirSystem:EquipmentList,
    OA Sys 1 Equipment,      !- Name
    Coil:Heating:Electric,   !- Component 1 Object Type
    Outdoor Air Preheat Coil,!- Component 1 Name
    HeatExchanger:AirToAir:SensibleAndLatent,  !- Component 2 Object Type
    OA Heat Recovery 1,      !- Component 2 Name
    OutdoorAir:Mixer,        !- Component 3 Object Type
    OA Mixing Box 1;         !- Component 3 Name

The fan and the cooling coil are listed in Branch.

  Branch,
    Air Loop Main Branch,    !- Name
    ,                        !- Pressure Drop Curve Name
    AirLoopHVAC:OutdoorAirSystem,  !- Component 1 Object Type
    OA Sys 1,                !- Component 1 Name
    Air Loop Inlet Node,     !- Component 1 Inlet Node Name
    Mixed Air Node,          !- Component 1 Outlet Node Name
    Fan:ConstantVolume,      !- Component 2 Object Type
    Supply Fan 1,            !- Component 2 Name
    Mixed Air Node,          !- Component 2 Inlet Node Name
    Cooling Coil Air Inlet Node,  !- Component 2 Outlet Node Name
    Coil:Cooling:Water:DetailedGeometry,  !- Component 3 Object Type
    Detailed Cooling Coil,   !- Component 3 Name
    Cooling Coil Air Inlet Node,  !- Component 3 Inlet Node Name
    Air Loop Outlet Node;    !- Component 3 Outlet Node Name