Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question

How to add Exhaust Fan to AirLoopHVAC:DedicatedOutdoorAirSystem

asked 2022-07-17 21:55:01 -0600

Keigo's avatar

updated 2022-10-12 21:23:54 -0600

I want to add Exhaust Fan to AirLoopHVAC:DedicatedOutdoorAirSystem. This is related to my previous question.

I could add HeatExchanger:AirToAir:SensibleAndLatent to AirLoopHVAC:DedicatedOutdoorAirSystem. Exhaust Fan is always on the exhaust side of the heat exchanger (EnergyPlus sometimes disregards it e.g., HVACTemplate:System:DedicatedOutdoorAir has Heat Recovery but does not have Exhaust Fan.), but I don't know how to add the Exhaust Fan.

image description

AirLoopHVAC:DedicatedOutdoorAirSystem is related to the following classes:

  • AirLoopHVAC:ControllerList
  • AirLoopHVAC:OutdoorAirSystem:EquipmentList
  • AirLoopHVAC:OutdoorAirSystem
  • AirLoopHVAC:Mixer
  • AirLoopHVAC:Splitter

If I add Fan:SystemModel at the end of AirLoopHVAC:OutdoorAirSystem:EquipmentList as shown below, I get a sever error.

image description

    ************* Beginning System Sizing Calculations
    ** Severe  ** The fan placement is either first as blow through or last as draw through inAirLoopHVAC:OutdoorAirSystem:EquipmentList = OA SUPPLY FAN
    **   ~~~   ** The current position is number 4
    **  Fatal  ** getAirLoopHVACDOAS: Previous errors cause termination.
    ...Summary of Errors that led to program termination:

AirLoopHVAC, Branch and BranchList are not used for AirLoopHVAC:DedicatedOutdoorAirSystem, so I think they are unrelated. I have no clue.

The idf file is here for your reference.

edit retag flag offensive close merge delete

1 Answer

Sort by » oldest newest most voted

answered 2022-07-18 03:57:12 -0600

It can indeed get quite confusing.

For the HX and Fan, this is actually exactly the same way you would do with a regular AirLoopHVAC: the HX and relief fan are going to be specified in relation to the AirLoopHVAC:OutdoorAirSystem. The weird thing is that your DOAS Coils and Fans are ALSO going to be specified in relation to the AirLoopHVAC:OutdoorAirSystem.

Another thing is that you won't have an OutdoorAir:Mixer (which makes it clearer which node is which node...).

Anyways, after using the right node connections, I can get a simulation to run properly (with some warnings, but I'm not trying to do something production ready here). Node connections is really the tricky bit, but OpenStudio SDK can help you do that, which is what I've done.

The other thing is is the order of the equipment on AirLoopHVAC:OutdoorAirSystem:EquipmentList matters a lot (the warning says so!). I chose to put the supply Fan first in line, and the relief last in line.

I have saved a working IDF file for you in this gist: . Search for "DOAS" (around line 1450), as I have been careful to name things properly.

The TL;DR is this:

  AirLoopDOAS,                            !- Name
  AirLoopDOAS OA Sys,                     !- AirLoopHVAC:OutdoorAirSystem Name
  Always On Discrete,                     !- Availability Schedule Name
  AirLoopDOAS Mixer,                      !- AirLoopHVAC:Mixer Name
  AirLoopDOAS Splitter,                   !- AirLoopHVAC:Splitter Name
  4.5,                                    !- Preheat Design Temperature {C}
  0.004,                                  !- Preheat Design Humidity Ratio {kgWater/kgDryAir}
  17.5,                                   !- Precool Design Temperature {C}
  0.012,                                  !- Precool Design Humidity Ratio {kgWater/kgDryAir}
  2,                                      !- Number of AirLoopHVAC
  LOOP1,                                  !- AirLoopHVAC Name 1
  LOOP2;                                  !- AirLoopHVAC Name 2

  AirLoopDOAS OA Sys,                     !- Name
  ,                                       !- Controller List Name
  AirLoopDOAS OA Sys Equipment List;      !- Outdoor Air Equipment List Name

  AirLoopDOAS OA Sys OA Inlet Node;       !- Node or NodeList Name 1

  AirLoopDOAS OA Sys Equipment List,      !- Name
  Fan:SystemModel,                        !- Component Object Type 1
  AirLoopDOAS Supply Fan,                 !- Component Name 1
  HeatExchanger:AirToAir:SensibleAndLatent, !- Component Object Type 2
  AirLoopDOAS HX,                         !- Component Name 2
  CoilSystem:Heating:DX,                  !- Component Object Type 3
  AirLoopDOAS HC CoilSystem,              !- Component Name 3
  CoilSystem:Cooling:DX,                  !- Component Object Type 4
  AirLoopDOAS CC CoilSystem,              !- Component Name 4
  Fan:SystemModel,                        !- Component Object Type 5
  AirLoopDOAS Relief Fan;                 !- Component Name 5

  AirLoopDOAS HX,                         !- Name
  AirLoopDOAS OA Sys OA Inlet Node,       !- Supply Air Inlet Node Name
  AirLoopDOAS OA Sys HX Outlet Node,      !- Supply Air Outlet Node Name
  AirLoopDOAS OA Sys Return Air Node,     !- Exhaust Air Inlet Node Name
  AirLoopDOAS OA Sys HX Relief Outlet Air Node, !- Exhaust Air Outlet Node Name
  Yes;                                    !- Economizer Lockout

  AirLoopDOAS HC CoilSystem,              !- Name
  Always On Discrete,                     !- Availability Schedule Name
  Coil:Heating:DX:SingleSpeed,            !- Heating Coil Object Type
  AirLoopDOAS HC;                         !- Heating Coil Name

  AirLoopDOAS HC,                         !- Name
  AirLoopDOAS OA Sys HX Outlet Node,      !- Air Inlet Node Name
  AirLoopDOAS HC Outlet to CC Inlet Node, !- Air Outlet Node Name
  2000;                                   !- Resistive Defrost Heater Capacity {W}

  AirLoopDOAS CC ...
edit flag offensive delete link more


Note that fan placement matters currently, but that's being addressed

ericringold's avatar ericringold  ( 2022-07-18 14:15:07 -0600 )edit

@Julien Marrec, I changed the order of the equipment in my idf file. No severe errors appered, but Air Mass Flow Rate [kg/s] of OA Supply Fan and Exhaust Fan became 0 all year around.

The flow rates remain 0 even if I hardsize the airflow rates of OA Suplly Fan, Exhaust Fan and HEX.

For the model without Exhaust Fan, Air Mass Flow Rate of OA Supply Fan and HEX is not 0.

I'm sure my node connection is fine.

How about the air flow rate of your model? For now, I think just changing the order of the equipment does not work.

Keigo's avatar Keigo  ( 2022-07-18 23:31:07 -0600 )edit

Since SmallOffice_CentralDOAS.idf is the only Example File which contains AirLoopHVAC:DedicatedOutdoorAirSystem, I'm following the Example File for the input method of the system. The Example File does not have a regular AirLoopHVAC and OutdoorAir:Mixer for the central DOAS system. That's why my model does not have them either. If there is another input method for central DOAS systems, I would appreciated a new Example File.

Keigo's avatar Keigo  ( 2022-07-18 23:40:01 -0600 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer


Question Tools

1 follower


Asked: 2022-07-17 21:55:01 -0600

Seen: 175 times

Last updated: Jul 18 '22