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OpenStudio HVAC Connections
OpenStudio uses a different method of connecting HVAC components compared to EnergyPlus. In EnergyPlus, two components are connected together using a unique node name.
For example in EnergyPlus a heating coil followed by a fan is connected via a unique node name, Node 20.
Coil:Heating:Fuel,
Coil Heating Gas 1, !- Name
Always On Discrete, !- Availability Schedule Name
NaturalGas, !- Fuel Type
0.8, !- Burner Efficiency
AutoSize, !- Nominal Capacity {W}
Node 19, !- Air Inlet Node Name
Node 20, !- Air Outlet Node Name
Node 20, !- Temperature Setpoint Node Name
0, !- Parasitic Electric Load {W}
, !- Part Load Fraction Correlation Curve Name
0; !- Parasitic Fuel Load {W}
Fan:ConstantVolume,
Fan Constant Volume 1, !- Name
Always On Discrete, !- Availability Schedule Name
0.7, !- Fan Total Efficiency
500, !- Pressure Rise {Pa}
AutoSize, !- Maximum Flow Rate {m3/s}
0.9, !- Motor Efficiency
1, !- Motor In Airstream Fraction
Node 20, !- Air Inlet Node Name
Node 12; !- Air Outlet Node Name
In OpenStudio, the fluid outlet of an HVAC component is joined to the inlet of another component via a OS:Connection object. The fluid inlets and outlets are called Ports in OpenStudio. The connection object references a source object and port combined with the target object and port, and in this way arbitrary connections can be made between any two components.
Unlike EnergyPlus, a Node is simply another component type in the HVAC topology. In the following example there are three components, a gas heating coil, followed by a node, followed by a fan.
OS:Coil:Heating:Gas,
{363b7008-0a0d-4fe7-9db3-68a79a48bff9}, !- Handle
Coil Heating Gas 1, !- Name
{3f88ddae-e2a9-41a1-a71b-ecde6608231c}, !- Availability Schedule Name
0.8, !- Gas Burner Efficiency
AutoSize, !- Nominal Capacity {W}
{dccdd7bc-d226-4808-9efa-90bdc10b94eb}, !- Air Inlet Port, port "5"
{f449861e-566d-4029-8f94-75a3714fe22c}, !- Air Outlet Port, port "6"
, !- Temperature Setpoint Node Name
0, !- Parasitic Electric Load {W}
, !- Part Load Fraction Correlation Curve Name
0; !- Parasitic Gas Load {W}
OS:Connection,
{f449861e-566d-4029-8f94-75a3714fe22c}, !- Handle
{7be3c785-24a5-42c4-b3d4-0e7066304acb}, !- Name
{363b7008-0a0d-4fe7-9db3-68a79a48bff9}, !- Source Object
6, !- Outlet Port
{048330b6-77f4-4087-b2f3-e0f09c368faf}, !- Target Object
2; !- Inlet Port
OS:Node,
{048330b6-77f4-4087-b2f3-e0f09c368faf}, !- Handle
Node 20, !- Name
{f449861e-566d-4029-8f94-75a3714fe22c}, !- Inlet Port, port "2"
{ec2a011e-f6aa-4857-a70b-e23706f580ea}; !- Outlet Port, port "3"
OS:Connection,
{ec2a011e-f6aa-4857-a70b-e23706f580ea}, !- Handle
{de2173c8-53af-4352-a30d-fc5a67f36184}, !- Name
{048330b6-77f4-4087-b2f3-e0f09c368faf}, !- Source Object
3, !- Outlet Port
{a0f6145b-064a-41c9-9c68-c63cb91aa3e5}, !- Target Object
8; !- Inlet Port
OS:Fan:ConstantVolume,
{a0f6145b-064a-41c9-9c68-c63cb91aa3e5}, !- Handle
Fan Constant Volume 1, !- Name
{3f88ddae-e2a9-41a1-a71b-ecde6608231c}, !- Availability Schedule Name
, !- Fan Efficiency
500, !- Pressure Rise {Pa}
AutoSize, !- Maximum Flow Rate {m3/s}
, !- Motor Efficiency
, !- Motor In Airstream Fraction
{ec2a011e-f6aa-4857-a70b-e23706f580ea}, !- Air Inlet Port, Port "8"
{d78d663b-7d54-41b1-acf0-1919b4e28ec0}, !- Air Outlet Port
; !- End-Use Subcategory
Here is a slightly more visual representation of the data, showing the relationships between the coil, node, and connecting data structure.
In most cases there is an OS:Node object between HVAC components, but it is not a requirement. There are a few examples where two OpenStudio HVAC Components connect without an intermediate node, and certainly the connection scheme is designed to be flexible enough to not require a node involved.
The Connection mechanism in OpenStudio involves more data relative to EnergyPlus. Typically this data is not authored directly, but instead connections are formed using OpenStudio's API.
m = OpenStudio::Model::Model.new
coil = OpenStudio::Model::CoilHeatingGas.new(m)
node = OpenStudio::Model::Node(m)
fan = OpenStudio::Model::FanConstantVolume.new(m)
m.connect(coil, coil.outletPort, node, node.inletPort)
m.connect(node, node.outletPort, fan, fan.inletPort)
OpenStudio clients do not generally use the connection API directly. Instead OpenStudio encourages higher level APIs, that use connect behind the scenes. With HVACComponent::addToNode, nodes and connections are formed automatically given the context of where an HVAC component is added.
m = OpenStudio::Model::Model.new
loop = OpenStudio::Model::AirLoopHVAC.new
coil = OpenStudio::Model::CoilHeatingGas.new(m)
fan = OpenStudio::Model::FanConstantVolume.new(m)
coil.addToNode(loop.supplyOutletNode())
fan.addToNode(loop.supplyOutletNode())
Currently ports are nothing but a field index, but they could be enhanced. For starters, ports would be better represented as names instead of indexes, and this improvement would flow naturally with a change from positional based osm format, to a key valued format. Secondly, there is currently only one type of port, a fluid connection, but it would be nice to have port "types" to allow different ports of the same type to be joined.