Branches, Connectors, and Nodes[LINK]

In EnergyPlus, the HVAC system and plant form a network (technically, a graph). The individual pieces of equipment - the fans, coils, chillers, etc. - are connected together by air ducts and fluid pipes. In EnergyPlus nomenclature, the air and fluid circuits are called loops. Specifying how an individual system and plant are connected is done in the EnergyPlus input (IDF) file. The overall structure of the network is defined with Branch and Connector objects. The detail is filled with components and their inlet and outlet nodes. A Branch consists of one or more components arranged sequentially along a pipe or duct. A Connector specifies how three or more branches are connected through a Splitter or Mixer. Nodes connect components along a branch: the outlet node of one component is the inlet node of the next downstream component. The nodes represent conditions at a point on a loop. Each component has one or more inlet and outlet nodes, depending on how many loops it interacts with. A fan, for instance, has one inlet node and one outlet node, since it interacts with a single air loop. A water coil will have 2 inlet and 2 outlet nodes, since it interacts with an air and a fluid loop. Figure 1 shows a diagram of an EnergyPlus HVAC input.

As an illustration of how such a network is built up on the IDF, here is the section of the IDF that describes the supply fan, splitter, and heating and cooling coil section of the dual duct air system.

    BranchList,
        Dual Duct Air Loop Branches,  !- Name
        Air Loop Main Branch,    !- Branch 1 Name
        Heating Coil Air Sys Branch,  !- Branch 2 Name
        Cooling Coil Air Sys Branch;  !- Branch 3 Name

    ConnectorList,
        Dual Duct Connectors,    !- Name
        Connector:Splitter,      !- Connector 1 Object Type
        DualDuctAirSplitter;     !- Connector 1 Name

    NodeList,
        Zone Equipment Inlet Node List,  !- Name
        Main Hot Air Inlet,      !- Node 1 Name
        Main Cold Air Inlet;     !- Node 2 Name

    NodeList,
        Air Loop Outlet Node List,  !- Name
        Heating Coil Outlet Node,!- Node 1 Name
        Cooling Coil Outlet Node;!- Node 2 Name

    Branch,
        Air Loop Main Branch,    !- Name
        autosize,                !- Maximum Flow Rate {m3/s}
        Fan:ConstantVolume,      !- Component 1 Object Type
        Supply Fan 1,            !- Component 1 Name
        Supply Fan Inlet Node,   !- Component 1 Inlet Node Name
        Supply Fan Outlet Node,  !- Component 1 Outlet Node Name
        PASSIVE;                 !- Component 1 Branch Control Type

    Branch,
        Heating Coil Air Sys Branch,  !- Name
        autosize,                !- Maximum Flow Rate {m3/s}
        Coil:Heating:Water,      !- Component 1 Object Type
        Main Heating Coil,       !- Component 1 Name
        Heating Coil Inlet Node, !- Component 1 Inlet Node Name
        Heating Coil Outlet Node,!- Component 1 Outlet Node Name
        ACTIVE;                  !- Component 1 Branch Control Type

    Branch,
        Cooling Coil Air Sys Branch,  !- Name
        autosize,                !- Maximum Flow Rate {m3/s}
        Coil:Cooling:Water,      !- Component 1 Object Type
        Simple Cooling Coil,     !- Component 1 Name
        Cooling Coil Inlet Node, !- Component 1 Inlet Node Name
        Cooling Coil Outlet Node,!- Component 1 Outlet Node Name
        ACTIVE;                  !- Component 1 Branch Control Type

    Connector:Splitter,
        DualDuctAirSplitter,     !- Name
        Air Loop Main Branch,    !- Inlet Branch Name
        Heating Coil Air Sys Branch,  !- Outlet Branch 1 Name
        Cooling Coil Air Sys Branch;  !- Outlet Branch 2 Name

    Fan:ConstantVolume,
        Supply Fan 1,            !- Name
        FanAndCoilAvailSched,    !- Availability Schedule Name
        0.7,                     !- Fan Efficiency
        600.0,                   !- Pressure Rise {Pa}
        autosize,                !- Maximum Flow Rate {m3/s}
        0.9,                     !- Motor Efficiency
        1.0,                     !- Motor In Airstream Fraction
        Supply Fan Inlet Node,   !- Fan Inlet Node Name
        Supply Fan Outlet Node;  !- Fan Outlet Node Name

    Coil:Cooling:Water,
        Simple Cooling Coil,     !- Name
        CoolingCoilAvailSched,   !- Availability Schedule Name
        autosize,                !- Design Water Flow Rate {m3/s}
        autosize,                !- Design Air Flow Rate {m3/s}
        autosize,         !- Design Inlet Water Temperature {C}
        autosize,         !- Design Inlet Air Temperature {C}
        autosize,         !- Design Outlet Air Temperature {C}
        autosize,     !- Design Inlet Air Humidity Ratio {kg-H2O/kg-air}
        autosize,     !- Design Outlet Air Humidity Ratio {kg-H2O/kg-air}
        Cooling Coil Water Inlet Node,  !- Water Inlet Node Name
        Cooling Coil Water Outlet Node,  !- Water Outlet Node Name
        Cooling Coil Inlet Node, !- Air Inlet Node Name
        Cooling Coil Outlet Node,!- Air Outlet Node Name
        SimpleAnalysis,          !- Type of Analysis
        CrossFlow;               !- Heat Exchanger Configuration

    AirLoopHVAC:ZoneSplitter,
        ZoneColdAirSupplySplitter,  !- Name
        Main Cold Air Inlet,     !- Inlet Node Name
        Zone 1 Dual Duct Cold Inlet,  !- Outlet 1 Node Name
        Zone 2 Dual Duct Cold Inlet,  !- Outlet 2 Node Name
        Zone 3 Dual Duct Cold Inlet;  !- Outlet 3 Node Name

    AirLoopHVAC:ZoneSplitter,
        ZoneHotAirSupplySplitter,!- Name
        Main Hot Air Inlet,      !- Inlet Node Name
        Zone 1 Dual Duct Hot Inlet,  !- Outlet 1 Node Name
        Zone 2 Dual Duct Hot Inlet,  !- Outlet 2 Node Name
        Zone 3 Dual Duct Hot Inlet;  !- Outlet 3 Node Name

    AirLoopHVAC:ZoneMixer,
        ZoneReturnAirMixer,      !- Name
        Return Air Mixer Outlet, !- Outlet Node Name
        Zone 1 Outlet Node,      !- Inlet 1 Node Name
        Zone 2 Outlet Node,      !- Inlet 2 Node Name
        Zone 3 Outlet Node;      !- Inlet 3 Node Name

    Coil:Heating:Water,
        Main Heating Coil,       !- Name
        FanAndCoilAvailSched,    !- Availability Schedule Name
        autosize,                !- U-Factor Times Area Value {W/K}
        autosize,                !- Maximum Water Flow Rate {m3/s}
        Heating Coil Water Inlet,!- Water Inlet Node Name
        Heating Coil Water Outlet,  !- Water Outlet Node Name
        Heating Coil Inlet Node, !- Air Inlet Node Name
        Heating Coil Outlet Node,!- Air Outlet Node Name
        UFactorTimesAreaAndDesignWaterFlowRate,  !- Performance Input Method
        autosize,                !- Nominal Capacity {W}
        82.2,                    !- Design Inlet Water Temperature {C}
        16.6,                    !- Design Inlet Air Temperature {C}
        71.1,                    !- Design Outlet Water Temperature {C}
        32.2;                    !- Design Outlet Air Temperature {C}

    Controller:WaterCoil,
        Main Cooling Coil Controller,  !- Name
        Temperature,             !- Control Variable
        Reverse,                 !- Action
        FLOW,                    !- Actuator Variable
        Cooling Coil Outlet Node,!- Sensor Node Name
        Cooling Coil Water Inlet Node,  !- Actuator Node Name
        0.001,             !- Controller Convergence Tolerance {deltaC}
        autosize,                !- Maximum Actuated Flow {m3/s}
        0.0;                     !- Minimum Actuated Flow {m3/s}

    Controller:WaterCoil,
        Main Heating Coil Controller,  !- Name
        Temperature,             !- Control Variable
        Normal,                  !- Action
        FLOW,                    !- Actuator Variable
        Heating Coil Outlet Node,!- Sensor Node Name
        Heating Coil Water Inlet,!- Actuator Node Name
        0.01,              !- Controller Convergence Tolerance {deltaC}
        autosize,                !- Maximum Actuated Flow {m3/s}
        0.0;                     !- Minimum Actuated Flow {m3/s}

Obviously, the creation of such a system/plant network description is best handled by a graphical user interface (GUI). However, for testing purposes a developer may have to create the input for a component by hand and insert it into an existing IDF. Then the developer must be careful to choose unique names for the branches and nodes and make sure the entire network makes physical sense.