EnergyPlus Airflow Network Modeling For Simple House

Introduction

This article discusses about basics of how AirflowNetwork is configured in EnergyPlus example file "AirflowNetwork_Simple_House.idf". The goal here is to discuss how nodes are defined and linked in AirflowNetwork and details about AirflowNetwork application are skipped. Air loop and Plant loop configuration in EnergyPlus are somewhat similar. AirflowNetwork, however, follows a different approach at times which adds to the confusion specially for the beginners and also for those who are familiar with Air loop and plant loop modeling. Basically, following two differences need to kept in mind between EnergyPlus Air/Plant loop configuration and AirflowNetwork configuration while creating AirflowNetwork model.

1. For AirflowNetwork, any of the following can be defined as a node

• Another node defined in AirLoopHVAC
• A thermal zone defined as Zone object and again defined through AirflowNetwork:MultiZone:Zone
• AirLoopHVAC:ZoneSplitter and AirLoopHVAC:ZoneMixer objects defined in AirLoopHVAC
• Other than above, AirflowNetwork nodes can also be defined by the user which are not defined in AirLoopHVAC

On the other hand, for air and plant loops, nodes are uniquely defined and another object is not used as a node. This difference in defining a node often times creates confusion when reviewing any AFN model.

2. For AirflowNetwork, same duct object can be used between different pair of nodes as shown in the example below. For air loops and plant loops, each pair of node has a unique object connecting them.

Example file: AirflowNetwork_Simple_House.idf

This example file has three thermal zones which are Living, Attic, and Garage. Only Living zone is conditioned. AirflowNetwork model has Supply duct located in Attic and return duct located in Garage.

It is recommended to simultaneously review EnergyPlus example file "AirflowNetwork_Simple_House.idf" along with diagrams below to understand the configuration of AirFlowNetwork in the file.

The Fig 1 below shows the demand and supply sides of the air loop of the file. The demand side has splitter and mixer with one inlet node and one outlet node. The constant volume no reheat terminal unit is serving the Living zone.

Fig 1: Airloop on simple residential building

AirflowNetwork configuration in the same file is shown in figure 2 below.

Fig 2: Airflow Network on simple residential building

Following points from above diagrams can help in creating an AirflowNetwork for an air loop

• All the nodes defined in air loop such as Zone Equipment Inlet Node and Return Air Mixer Outlet etc. need to be redefined in AirflowNetwork (using AirflowNetwork:Distribution:Node object) as shown in Fig 1 above. These AirflowNetwork nodes will then be used to establish the linkage between them.
• AirFlowNetwork allows ducts to be modeled between the following nodes in air loops (supply outlet & demand inlet nodes) and (supply inlet & demand outlet nodes).
• Same duct object can be used as connection component in different node pairs. For example as Fig 2 shows, duct named ZoneSupply is a component between the following two node pairs 1)  ZoneSupplyNode & ZoneSupplyRegisterNode ATInlet, and 2) SplitterNode & ZoneSupplyNode. Similarly other ducts such as ZoneConnectionDuct, ZoneReturn and MainReturn duct are used as component between different pair of nodes.
• Nodes which are not defined in AirLoopHVAC and only defined in AirFlowNetwork such as ZoneSupplyNode and ZoneReturnNode are used in this example file to model supply leak and return leak respectively.
• Air loop components such as fan, cooling coil and heating coil need to be redefined in AirFlowNetwork.
• Once the AFN nodes are defined, they can be connected through AirflowNetwork:Distribution:Linkage. If the component which connects the nodes is a duct, then this object also allows the specification of the thermal zone where the ducts are located.

If not starting from an existing EnergyPlus example file, it is a good idea to draw schematic of air loop and AFN with associated nodes. This can save time when finally configuring the AFN model in EnergyPlus and will also help later during debugging.